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10.1186/s11671-024-04101-w
Research
Conductivity optimisation of graphene oxide-M13 bacteriophage nanocomposites: towards graphene-based gas micronano-sensors
Stokes Kate 1
Sun Yiwei yiwei.sun@qmul.ac.uk

12
Thomas Jarrod L. 1
Passaretti Paolo 3
White Henry 4
Goldberg Oppenheimer Pola GoldberP@bham.ac.uk

15
1 https://ror.org/03angcq70 grid.6572.6 0000 0004 1936 7486 School of Chemical Engineering, Advanced Nanomaterials Structures and Applications Laboratories, College of Engineering and Physical Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT UK
2 grid.521132.6 Paragraf Limited, Cambridge, PE28 3EB UK
3 https://ror.org/03angcq70 grid.6572.6 0000 0004 1936 7486 Institute of Cancer and Genomic Sciences, University of Birmingham, Birmingham, B15 2TT UK
4 grid.1343.5 0000 0004 0421 9667 BAE-Systems - Air Sector, Buckingham House, FPC 267, Filton, Bristol, UK
5 Healthcare Technologies Institute, Institute of Translational Medicine, Mindelsohn Way, Birmingham, B15 2TH UK
18 9 2024
18 9 2024
12 2024
19 1 15219 6 2024
2 9 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by/4.0/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Graphene oxide (GO) and M13 bacteriophage can self-assemble to form ultra-low density porous structures, known as GraPhage13 aerogels (GPA). Due to the insulating nature of GPA and the challenges in producing highly conductive aerogels, it is paramount to explore ways to enhance the conductivity of GPA. Herein, we have developed a method to enhance the conductivity of GPA, via the integration and optimisation of 5 nm and 20 nm diameter gold nanoparticles (AuNPs) into the aerogel structure and systematically analysed the morphology, composition and spectroscopic properties of the resulting GPA-Au nanocomposite. The fabricated GPA-Au nanocomposites exhibited remarkable increases in conductivity, with the integration of 5 nm AuNPs leading to a 53-fold increase compared to GPA, achieving a performance of up to 360 nS/cm, which is within the range suitable for miniaturised semiconductor devices. The mechanism behind the conductivity enhancement was further investigated and attributed to GO-AuNP interactions increasing the carrier density by introducing new energy levels in the GO band gap or shifting its Fermi level towards the conduction band. These findings demonstrate the potential of functionalised AuNPs to significantly improve the electrical properties of GPA, paving the way for their application in gas sensors for biological and chemical detection and a new range of advanced semiconductor devices.

Keywords

Graphene oxide
M13 bacteriophage
Gold nanoparticles
Conductivity optimisation
BAE SystemsBS/1464085 BS/1464085 White Henry Goldberg Oppenheimer Pola EPSRCEP/V029983/1 and EP/W004593/1 Goldberg Oppenheimer Pola http://dx.doi.org/10.13039/100010269 Wellcome Trust 174ISSFPP Goldberg Oppenheimer Pola issue-copyright-statement© Springer Science+Business Media, LLC, part of Springer Nature 2024
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pmcIntroduction

Graphene is a monolayer of sp2-hybridised carbon atoms arranged in a honeycomb lattice, exhibiting remarkable properties including high carrier mobility, mechanical strength as well as electrical and thermal conductivity [1]. Despite these advantages, its widespread utilisation has been impeded by various challenges including the scalability issues and chemical inertness [2]. Graphene-based derivatives, such as graphene oxide (GO), offer a solution to these limitations. GO is comprised of oxygen-containing functional groups (OCFGs) including for instance, hydroxyl, carboxyl, carbonyl and epoxide, which enable the GO with the versatile capacity to interact with various compounds [3]. Notably, it can interact with biomolecules, leading to the formation of graphene-based bionano-composites, which have found applications in diverse fields such as energy storage, absorbents and drug delivery [4–7].

Recent research has demonstrated GO can interact with the M13 bacteriophage, a filamentous virus with dimensions of approximately 6.6 nm width and 880 nm length [8]. M13 is comprised of circular single-stranded DNA encapsulated by 2700 copies of the pVIII major coat protein, and the pIII, pVI, pVII and pIX minor coat proteins [9]. The interactions between GO and M13 initiate their self-assembly into a GraPhage13 hydrogel (GPH), which is subsequently dried in a vacuum to fabricate GraPhage13 aerogels (GPA), porous nanocomposites with an ultra-low density of 8.8 mg/cm3 and high surface area of 325 m2/g. The tunability of the GPA properties, either through the functionalisation of its constituents or interactions with the OCFGs in GO and chemical/genetic modification of M13, enable the routes for tailoring GPA for specific applications. With its scalable, environmentally friendly and cost-effective production, GPA is establishing itself as a versatile nanomaterial for a broad range of applications including for instance, composite scaffolds, absorbers and sensors [10, 11].

The development of highly conductive graphene-based aerogels is imperative given their immense applied potential for energy storage [12, 13], pressure [14, 15] and gas sensors [16, 17]. However, achieving high conductivity in these aerogels remains challenging. Their inherently porous nature often results in an amorphous structure with numerous defects, impeding efficient electron transfer within the material [18]. Additionally, in GPAs, the OCFGs in GO act as scattering sites, reducing the carrier mobility and leading to a low conductance of approximately 0.2 nS [19, 20]. This limitation restricts the applications of GPA in various fields, such as gas sensing, where the sensor sensitivity relies on changes in electrical conductivity caused by interactions between the sensor and the analyte [21]. Semiconducting materials within gas sensors typically exhibit conductivities ranging from 10−7 to 102 S/cm, ensuring the detection of small conductivity changes for effective identification of gas-based biological and chemical analytes [22, 23]. However, through the functionalisation of GO and M13, it is possible to incorporate various nanomaterials into the GPA structure, offering means to modulate the conductivity. Previous research has demonstrated a 30-fold increase in its conductance though the incorporation of carbon nanotubes, highlighting the potential of this approach to produce conductive graphene-based aerogels [20].

In this study, we develop and establish a route to enhance the conductivity of GPA by integrating gold nanoparticles (AuNPs) into its micronano-structure. The interaction between GO, M13 and carboxylic acid functionalised AuNPs with diameters of 5 nm and 20 nm were investigated with ultraviolet–visible (UV–Vis) spectroscopy and the composition optimised via energy-dispersive X-ray (EDX) spectroscopy. Changes in the morphology and microstructure of the GPAs with integrated 5 nm AuNPs (GPA-Au5nm) and 20 nm AuNPs (GPA-Au20nm) were observed with scanning electron microscopy (SEM) and Raman spectroscopy, and their I-V characteristics were measured to determine if the integration of AuNPs significantly enhances conductivity. It was found that the AuNPs formed electronic interactions with GO [24, 25], enabling their integration within GPA, with the concentration of AuNPs within GPH saturating at 0.59 ± 0.01 mg/ml. The resulting GPA maintained its porous micronano-structure, and a discrete distribution of AuNPs within the optimised GPA-Au20nm was evident via the SEM imaging. The insulating nature of GPA was initially confirmed with a typical conductance of 0.34 ± 0.02 nA and conductivity of 6.8 nS/cm. The integration of 20 nm and 5 nm AuNPs into GPA yielded increased conductivities of 1.9 × 10–7 S/cm and 3.6 × 10–7 S/cm, respectively, well within the range of semiconducting materials in gas sensors. SEM and Raman spectral analysis revealed that the mechanism behind the enhanced conductivity is due to the formed electronic interactions between GO and AuNPs, which introduce new energy levels in the GO band gap or shift its Fermi level towards the conduction band, thereby increasing carrier concentration.

These results further demonstrate the tunability of GraPhage13 aerogels, paving the way for the integration of additional nanomaterials to enhance their properties for specific applications. The significant improvement in GPA conductivity achieved through the incorporation of gold nanoparticles underscores the potential of these materials for advanced technologies. By enhancing conductivity while maintaining structural integrity, this study advances the understanding of nanomaterial interactions within aerogels and lays a solid foundation for the development of high-performance, multifunctional materials. The demonstrated conductivity improvements bring GPAs closer to practical use in semiconducting sensor devices, particularly for gas-based bio- and chemical sensors, with applications in diagnostics, environmental monitoring and detection of explosives [26, 27]. Furthermore, these findings emphasise the potential for further exploration of integrating additional nanomaterials into GPAs to tailor the intrinsic properties such as various types of nanoparticles to achieve even greater conductivity and other functional properties. The overall insights gained from this study provide constructive guidance, paving the way toward realising the full potential of graphene-based aerogels across a wide range of applications.

Results and discussion

Incorporation of gold nanoparticles

The interactions between AuNPs and GPAs were investigated via the UV–Vis spectroscopy. In the initial stages of GPA production, GO and M13 are introduced into a buffer, enabling their self-assembly. Following this, centrifugation separates and isolates the components from solution. Through removing 90% of the supernatant and re-suspending the pellet, the GraPhage13 hydrogel (GPH) is generated. However, the supernatant itself provides valuable insights. By combining GO and AuNPs (GO-AuNPs), M13 and AuNPs (M13-AuNPs) and all the components (GPH-AuNPs), followed by centrifugation, supernatant analysis and comparison with the UV–Vis spectra of the individual components within the solution, we determine whether the components interacted or eliminated from the solution or whether they remain unreactive and remain present in solution (Fig. 1).Fig. 1 UV–Vis spectra of the supernatants generated from mixtures of graphene oxide (GO), M13 bacteriophage and carboxylic acid functionalised gold nanoparticles (AuNPs) with diameters of 5 nm and 20 nm. The spectrum of GO, M13 and GPH supernatants in comparison to the addition of the GO and M13 spectra, are shown in (ai) with (aii) zoom-in on the features of the GO and GPH spectra. b The supernatants produced by centrifuging 5 nm and 20 nm AuNPs. c Comparison of the addition of M13 and Au supernatants (M13 + AuNPs) after mixing M13 and Au (M13-AuNPs) with 5 nm and 20 nm AuNPs. d The supernatants from the addition and mixtures of graphene oxide (GO) with 5 nm and 20 nm AuNPs and e the UV–Vis spectrum of mixing GraPhage13 hydrogel (GPH) and 20 nm and 5 nm AuNPs

The UV–Vis spectra (Fig. 1a) illustrate the characteristics of the supernatants produced after centrifugation of GO, M13 and GPH, compared to the combined spectra of GO and M13 (GO + M13). In the GO supernatant spectrum, a peak at 230 nm indicates the π → π* transition of C=C bonds, while a shoulder at 310 nm suggests n → π* transitions of C=O bonds [28]. The M13 spectrum exhibits a prominent absorption band between 200–230 nm, attributed to the π → π* transitions of peptide bonds, with a distinct peak at 269 nm resulting from the major coat protein pVIII and viral DNA [10]. The GPH supernatant spectra resembles the spectrum for GO with a more pronounced peak at 269 nm due to the presence of the M13. When comparing GPH to GO + M13, the absorption notably decreases, e.g., the peak at 269 nm decreases from 1.1 to 0.12. This indicates that the interactions between GO and M13, namely the pH-dependent electrostatic interactions between the negatively charged carboxylic acids on GO and the positively charged groups of the N-terminus and K8 residues of the M13, produce GO-M13 pellet, which precipitates from solution to produce GPH [8, 10].

The UV–Vis spectra of 5 nm and 20 nm AuNPs supernatants (Fig. 1b) display characteristic peaks around 524 nm and 522 nm, respectively, corresponding to localised surface plasmon resonance. While both AuNPs exhibit similar absorption behaviour, their interaction with M13 is negligible, as indicated by consistent M13 peak absorbance in the presence and absence of AuNPs (Fig. 1c). Conversely, the combination of GO and AuNPs leads to a significant reduction in AuNP absorption (Fig. 1d), indicating their interaction and subsequent precipitation [29]. Similarly, AuNPs integrate into GPH, resulting in decreased absorption peaks for both AuNPs and GPH components compared to their individual spectra. This reduction in absorbance upon mixing with GPH signifies the interactions between AuNPs and GO, facilitating their precipitation from the solution. This hybridisation, involving the carboxyl groups on the AuNPs and the oxygen-containing functional groups on GO [24, 25], creates a new composite material where the electronic and structural properties of GO are significantly modified by the presence of AuNPs.

Optimisation of GPA-Au

To determine the optimal concentration of the 5 nm and 20 nm AuNPs for integration into GPA, GPH-Au with varying concentrations of AuNPs were synthesised and the resulting GPA-Au analysed using energy dispersive x-ray spectroscopy (EDX). The weight percentage of gold incorporated into GPA was studied across different AuNP concentrations (Fig. 2a).Fig. 2 a The relationship between the concentration of gold nanoparticle (AuNPs) solution utilised to produce the GraPhage13 hydrogel (GPH), subsequently dried to form the GraPhage13 aerogel (GPA) and the resulting weight percentage of Au in the aerogel containing AuNPs with diameters of 5 nm (GPA-Au5nm) and 20 nm (GPA-Au20nm). b Representative EDX spectrum of GPA-Au

As the concentration of AuNPs increases, the wt% Au also increases until reaching a plateau, indicating saturation of GPH and no further increase in AuNP concentration within GPA. For GPA-Au20nm, saturation occurs at approximately 0.61 mg/ml, resulting in 50 ± 2 wt% Au, while for GPA-Au5nm, saturation occurs at approximately 0.58 mg/ml, yielding 56 ± 1 wt% Au. The higher maximum wt% Au for GPA-Au5nm compared to GPA-Au20nm may be attributed to the larger size of 20 nm AuNPs blocking some of the binding sites on GO, resulting in lower Au content. Additionally, the smaller 5 nm AuNPs may integrate more effectively within the micro-nanostructure. A representative spectrum of optimised GPA-Au is shown in Fig. 2b, with SEM images of resulting GPA-Au in Fig. 3, where the presence of 20 nm AuNPs is clearly visible (Fig. 3e–f).Fig. 3 SEM images of GraPhage13 aerogels functionalised with a–b 5 nm AuNPs and c–f 20 nm AuNPs

In-depth Raman spectroscopy

From our previous Raman analyses of GPA [20, 30], at least two distinctive peaks were expected, the D-mode, which has contributions from both defective sp2 carbon, and sp3 amorphous or disordered carbon, and the G-mode, stemming from the sp2 bond stretching. Other peaks which may be present for GPA are the G− and D’ modes. The G− mode typically appears a few tens of wavenumbers lower than the G mode and is associated with significantly modified sp2 bonds. In GPA, this mode could arise from consistently softened sp2 bonds, such as those resulting from intensive functionalisation in GO, or from the bent graphene planes after its assembly with M13. Additionally, it may result from charge transfer between GO and M13, leading to a reduction in the energy of a portion of transverse optical (TO) or longitudinal optical (LO) phonons. The D’ mode arises from crystal defects due to sp2 bond functionalisation in GO [20, 30].

Herein, Raman spectroscopy was conducted on GPA, GPA-Au5nm and GPA-Au20nm using excitation wavelengths of 514 nm and 633 nm. The purpose of using two lasers was to identify whether the observed Raman features in the D mode range originated from the D mode associated with C–C sp2 bonds or the sp3 Raman mode, as the D mode frequency shifts with laser excitation, whereas a sp3 Raman peak is non-dispersive peak [31–33]. The Bayesian information criterion (BIC) was utilised to determine the optimal objective models for fitting each spectrum (Fig. 4) [20].Fig. 4 Raman spectra of GPA, GPA-Au5nm and GPA-Au20nm at wavelengths of 514 nm and 633 nm, with the overall curve fit. The Lorentzian and Gaussian fits are given by dashed and dotted lines, respectively

Under 514 nm excitation, the D features of all three samples consist of a single peak at 1355 cm−1 (Table 1), despite changes in peak shape and frequency shifts. This peak is approximately 20 cm−1 higher than the sp3 peak, with its expected Raman shift of 1332 cm−1, hence attributed to the D mode. The optimal peak fit for the D-mode was generated by fitting one Lorentzian and one Gaussian peak, termed DL and DG respectively, at the same Raman shift of 1355 cm−1, generating a mixed Gaussian–Lorentzian line-shape [34, 35].Table 1 Raman shift, full width half maximum (FWHM) and intensity of the Raman peaks present in spectra of GPA, GPA-Au5nm and GPA-Au20nm, obtained at a wavelength of 514 nm

Peak	GPA	GPA-Au5nm	GPA-Au20nm	
Raman Shift
(cm−1)	FWHM (cm−1)	Intensity
(cm−1)	Raman Shift
(cm−1)	FWHM (cm−1)	Intensity
(cm−1)	Raman Shift
(cm−1)	FWHM (cm−1)	Intensity
(cm−1)	
DL	1356.5 ± 0.4	39 ± 2	(1.1 ± 0.1) × 106	1354.6 ± 0.3	50 ± 2	(7.0 ± 0.4) × 105	1355.7 ± 0.3	48 ± 2	(9.3 ± 0.6) × 105	
DG	62 ± 1	(9.6 ± 0.6)

 × 103

	74 ± 1	(2.9 ± 0.2) × 103	73 ± 2	(3.3 ± 0.2) × 103	
G−	1539 ± 3	44 ± 3	(1.1 ± 0.1) × 106	1553 ± 4	62 ± 3	(8.2 ± 0.8) × 105	1550 ± 2	53 ± 1	(8.2 ± 0.2) × 105	
G	1598.3 ± 0.6	26.3 ± 0.5	(1.84 ± 0.03) × 103	1602.4 ± 0.5	24.1 ± 0.5	(6.3 ± 0.2) × 103	1601.6 ± 0.4	24.5 ± 0.4	(8.2 ± 0.1) × 103	

Under 633 nm excitation, GPA also exhibits a single D peak similar to that observed under 514 nm, however its position downshifts to 1338.7 cm−1. The D mode is activated by a defect-induced double resonance process, involving the energy of the LO phonon at the K point in the Brillouin zone. The presence of a Kohn anomaly at the K point causes the highly dispersive nature of the D peak [36].

The splitting of the D profile into three peaks, D1-3, is clearly visible for GPA-Au20nm under 633 nm and becomes evident for GPA-Au5nm through comparing BIC values. This split appears following the introduction of AuNPs and is more pronounced with 20 nm AuNPs. Meanwhile, this split does not appear under 514 nm. These observations indicate that the split peaks are likely modified D peaks, becoming visible due to being in resonance under 633 nm. The integration of AuNPs into GPA modifies the sp2-network, altering the phonon dispersion curve, particularly the LO branches. This results in a significant shift in the G− mode (LO at the Gamma point) (Tables 1–2) and an obvious splitting of the D mode (LO at the K point), as captured by the 633 nm laser. The modification of the dispersion curve, indicative of perturbations to the sp2 network of GPA, is more substantial with 20 nm AuNPs than with 5 nm AuNPs [37, 38].Table 2 Raman shift, full width half maximum (FWHM) and intensity of the Raman peaks for GPA, GPA-Au5nm and GPA-Au20nm, obtained at a wavelength of 633 nm

Peak	GPA	Peak	GPA-Au5nm	GPA-Au20nm	
Raman Shift
(cm−1)	FWHM (cm−1)	Intensity
(cm−1)	Raman Shift
(cm−1)	FWHM (cm−1)	Intensity
(cm−1)	Raman Shift
(cm−1)	FWHM (cm−1)	Intensity
(cm−1)	
DL	1338.7 ± 0.5	38 ± 2	(7 ± 1) × 105	D1	1229 ± 4	32 ± 4	(6.4 ± 0.4) × 105	1239.5 ± 0.8	11 ± 1	(1.9 ± 0.3) × 105	
DG	53.7 ± 0.8	(1.1 ± 0.1) × 104	D2	1335 ± 2	33 ± 8	(1.1 ± 0.6) × 106	1321 ± 1	40 ± 1	(3.0 ± 0.2) × 106	
-	-	-	-	D3	1374 ± 8	50 ± 4	(1.1 ± 0.4) × 105	1372.5 ± 0.5	26.6 ± 0.9	(2.1 ± 0.1) × 106	
G−	1522 ± 3	64 ± 4	(9.6 ± 0.8) × 106	G−	1537 ± 4	53 ± 5	(9 ± 1) × 105	1501 ± 2	58 ± 3	(1.8 ± 0.1) × 106	
G	1598.3 ± 0.3	27.0 ± 0.4	(1.4 ± 0.2) × 104	G	1600.5 ± 0.5	25.1 ± 0.4	(1.52 ± 0.03) × 104	1597.3 ± 0.3	28.6 ± 0.4	(2.44 ± 0.03) × 104	

Additionally, the 633 nm Raman spectrum of GPA-Au20nm shows the highest peak intensities, with a maximum intensity of (3.7 ± 0.2) × 104 counts, in comparison to (1.9 ± 0.1) × 104 counts for GPA-Au5nm. This might be attributed to the localised surface plasmon resonance (LSPR) exhibited by AuNPs. When electromagnetic radiation interacts with the conduction electrons on the surface of the AuNPs, it causes the electrons to oscillate at the same frequency as the incident light. At the resonant frequency, this results in an amplified local electromagnetic field, leading to an increased Raman signal [39]. For AuNPs with diameters smaller than 50 nm, collisions between the electrons and the particle surface decreases their mean free path. This dampens the LSPR proportionally to the particle diameter, thereby reducing the Raman intensity [40].

To gain further insight into Raman spectra, the intensity ratio of D to G peaks (ID/IG) was calculated (Table 3), to give a measure of the concentration of defects and disorder within a sample [41].Table 3 The intensity ratio of D-peak to G-peak (ID/IG) of Raman peaks for GPA, GPA-Au5nm and GPA-Au20nm, at wavelengths of 514 nm and 633 nm

Parameter	GPA	GPA-Au5nm	GPA-Au20nm	
514 nm	633 nm	514 nm	633 nm	514 nm	633 nm	
ID/IG	0.91 ± 0.08	0.81 ± 0.08	0.86 ± 0.07	3.1 ± 0.6	1.15 ± 0.05	2.9 ± 0.1	

At the 633 nm laser excitation, the integration of AuNPs into GPA increases ID/IG due to an increase in the intensity of the D-profile, as many modified D modes become resonant. This D-profile splitting indicates that the AuNPs introduce defects into the structure, likely resulting from interactions between GPA and the AuNPs [42, 43]. Furthermore, ID/IG appears to be independent of the diameter of the AuNPs. This may be attributable to the GPA-Au composites being produced using the same fabrication process, ensuring similar chemical and structural properties. Additionally, the defects are intrinsic to the GPA structure and the interactions between GO and the carboxylic groups on the AuNPs, making them independent of the AuNP diameter. Moreover, the increase in ID/IG through the integration of AuNPs suggests an increase in the electrical conductivity [44]. These trends are not observed at the 514 nm laser excitation. There is little change in ID/IG between GPA and GPA-Au5nm, and only a slight increase for GPA-Au20nm. The 514 nm laser is not able to induce resonance in the modified D modes and therefore, structural changes to GPA due to the presence of AuNPs are less detectable [37].

Conductivity of GPA and GPA-Au

The I-V characteristic of GPA (Fig. 5a) aligns closely with previous findings [20] and confirms the insulating nature of GPA, with a maximum current of 3.4 ± 0.2 nA at 10 V. This leads to a conductance of 0.34 nS and a conductivity of 6.8 nS/cm, two orders of magnitude below the minimum conductivity for semiconducting materials utilised within gas sensors. GPA-Au20nm (Fig. 5b) exhibits a significantly enhanced conductivity, reaching 90 ± 8 nA at 10 V, marking a 26-fold increase over GPA and generating a conductivity of 190 nS/cm, achieving a value within the range for semiconducting materials in gas sensors. The conductivity was enhanced further through the integration of 5 nm AuNPs, with GPA-Au5nm demonstrating a current of 180 ± 4 nA at a bias of 10 V, twice that of GPA-Au20nm and representing a 53-fold increase compared to GPA. This is equivalent to a conductance of 18 nS and a conductivity of 360 nS/cm, representing a significant step towards the integration of GPA in sensing devices.Fig. 5 a I–V characteristic of GPA and b comparison of the I–V characteristics of GPA, GPA-Au20nm and GPA-Au5nm

Through the morphological studies via the SEM and microstructural changes via Raman spectroscopy, the mechanism by which the conductivity of GPA is enhanced through the integration of AuNPs has been subsequently determined. Since SEM images demonstrate a discrete distribution of AuNPs across the GPA, the current must be transmitted through GPA itself, rather than exclusively through the AuNPs. Additionally, since concentration of 5 nm and 20 nm AuNPs introduced into GPH was equal, and yet the GPA-Au5nm proved to be twice as conductive as GPA-Au20nm, the presence of Au cannot be the sole cause of the increased conductivity. Therefore, the conductivity enhancement must be through the interactions between GO-M13 and the AuNPs. The greater increase in conductivity observed for the integration of 5 nm AuNPs compared to the 20 nm AuNPs is likely due to the 5 nm AuNPs possessing a higher surface-to-volume ratio, facilitating a higher surface area for interactions between GO and the AuNPs [45].

There are two mechanisms by which the AuNPs interacting with GO-M13 could enhance the conductivity of the resulting GPA, either by increasing the carrier density or by increasing the carrier mobility [46]. The presence of scattering at functional sites within GO significantly reduces its carrier mobility. Reducing GO to reduced graphene oxide eliminates some of the functional groups and aids in the dispersion of the free carriers, increasing their mobility [19].

This would be evident in Raman spectra via a redshift in the G-mode position as the concentration of sp2 bonds increases and a reduction in ID/IG resulting from a reduced level of defects [47]. Similarly, the bonding of the AuNPs to GO would reduce the concentration of available functional groups and therefore the G-mode position and ID/IG would be expected to decrease. Conversely, a significant increase in ID/IG is observed and the G-mode remains relatively constant when comparing GPA to GPA-Au5nm and GPA-Au20nm. Therefore, an increase in the carrier density is most probable mechanism behind the conductivity enhancement. An increase in the carrier density through the introduction of AuNPs into GPA likely originates from alterations to the band gap of GO, which has a relatively wide band gap of 2.2 eV, rendering GO an insulator [48, 49]. The presence of AuNPs introduces new energy states within this band gap, shift the Fermi level towards the conduction band, or a combination of these effects, resulting in an increased conductivity [50–53].

Smaller AuNPs possess a higher surface-to-volume ratio, which increases the available surface area for interaction with GO, leading to a more stable dispersion of the AuNPs within the solution. This stable dispersion promotes a higher density of GO-Au interactions, which is essential for enhancing the carrier density through the previously discussed mechanism, thereby improving the conductivity. Consequently, GPA-Au5nm exhibits a higher conductivity compared to GPA-Au20nm [54, 55].

Given the mechanism by which AuNPs enhance the conductivity of GPA, a positive correlation between the amount of AuNPs incorporated into GPA and the resulting conductivity is expected. The conductivity improvement depicted in Fig. 5 corresponds to the maximum saturation of Au wt% in GPA. This implies that increasing the AuNP loading beyond this saturation point will not result in further conductivity gains. Conversely, reducing the AuNP load decreases the Au wt% in GPA, thereby diminishing the conductivity enhancement. If the AuNP load is sufficiently reduced, the GPA will eventually return to its insulating state in the absence of AuNPs.

Conclusions

A novel method has been developed and validated for transforming insulating graphene oxide-based aerogels into conductive materials suitable for semiconductor sensors. This advancement paves the way for future integration into electronic systems and the development of advanced graphene-based sensors. The integration of 5 nm and 20 nm diameter carboxylic acid-functionalised gold nanoparticles (AuNPs) into GraPhage13 aerogels (GPA) was systematically investigated using UV–Vis spectroscopy and EDX. UV–Vis spectra revealed interactions between graphene oxide (GO) and AuNPs, facilitating their integration into the GraPhage13 hydrogel (GPH), the precursor for GPA. EDX measurements determined the saturation concentration of AuNPs within GPH. The optimal GPA-Au with the maximum concentration of AuNPs was further analysed using SEM, Raman spectroscopy, and conductivity measurements. SEM analysis confirmed the integration of AuNPs into the GPA micro-nanostructure, with Raman spectra showing distinct changes in the D-mode and G-mode due to the presence of AuNPs. These changes were more pronounced in GPA-Au20nm compared to GPA-Au5nm, indicating that AuNPs modify the sp2 carbon network and phonon dispersion in GO. Conductivity measurements revealed a significant increase in GPA conductivity, transitioning from an insulator with a conductivity of 6.8 nS/cm to 190 nS/cm and 360 nS/cm for GPA-Au20nm and GPA-Au5nm, respectively. This study highlights the potential to tune GPA properties through nanoparticle integration, offering possibilities for further customisation with various nanomaterials and modifications of M13 viral building blocks. The significant enhancement of GPA conductivity through AuNP integration makes it a promising candidate for integration into miniaturised devices for applications such as energy storage, gas, and pressure sensors.

Materials and methods

Propagation and purification of M13 bacteriophage

The method is described by Stokes et al. [8]. Briefly, One Shot TOP10F’ Chemically Competent Escherichia coli (E. coli) (Thermo Fisher Scientific) cells were inoculated on nutrient agar plates and subsequently, incubated overnight at 37 °C. The cultivated cells were depositing into 50 ml falcon tubes with nutrient broth (NB) and tetracycline in ethanol (Sigma) (TCN), to a final concentration of 5 μg/ml. The tubes were incubated overnight in a shaker incubator at 37 °C, 150 rpm and then added to further NB and TCN in conical flasks. M13 (0.05 mg) from a stock solution was incubated overnight in shaker incubator. The solution was centrifuged twice (Beckman Coulter, JLA 10.5) to remove the E. coli cells, mixed with a 25% polyethylene glycol (PEG) 6000 and 2.5 M NaCl (PEG-NaCl) and stirred on ice for 90 min. The solution was centrifuged to produce a while pellet, which was resuspended in DIW and centrifuged in a microcentrifuge (SciSpin MICRO). The addition of PEG-NaCl and leaving the solution on ice for 60 min enabled the M13 to precipitate from solution and centrifuging yielded the M13 pellet, resuspended in DIW.

Fabrication of GraPhage13 aerogels

The methodology is described by Passaretti et al. [10]. M13 and GO (Graphene Supermarket, SKU-HCGO-W-175ML) were introduced into a 10 mM citrate buffer with a pH of 4.9, both reaching a concentration of 0.3 mg/ml. Subsequently, the solution was thoroughly mixed using an orbital shaker at 150 rpm for 15 min, followed by centrifugation at 15,000 rpm for 1 min. This step yielded the formation of a GO and M13 pellet. By eliminating 90% of the supernatant and re-suspending the pellet, GraPhage13 hydrogel (GPH) materialises. The next phase included depositing 100µL of GPH onto a cleaned glass substrate, subsequently subjected to vacuum drying for 60 min, culminating in the creation of the GPA product. For the fabrication of GPA-Au, 250 μL of 5 nm (765430-1ML, core diameter 3-7 nm) or 20 nm (765511-1ML, core diameter 18–22 nm) carboxylic acid functionalised gold nanoparticles (Sigma-Aldrich) were introduced into the citrate buffer with GO and M13.

UV–Vis spectroscopy

An Aligent Cary 60 UV–Vis spectrophotometer was used to determine the concentration of M13 [8]. Briefly, a baseline spectrum was taken in quartz cuvette with 1 cm path length and the M13 in DIW was analysed. The characteristic spectrum of M13 presented a local minimum in absorbance at 245 nm, a local maximum at 269 nm and a baseline at 350 nm. The concentration of M13 was calculated with the Beer-Lambert Law and an extinction coefficient of 3.84cm2/mg at 269 nm [56]. UV–Vis was also employed to analyse the supernatants of GO, M13 and AuNPs to determine whether the components had interacted and precipitated from solution or remained in the supernatant. To replicate the conditions required to produce GraPhage13 aerogels, the supernatants for GO and/or M13 were generated (Sect. 4.2) but instead the supernatant was deposited into quartz cuvette and spectra recorded. For producing solutions of AuNPs, 10ul of citrate buffer was removed and 10ul of AuNPs added.

Scanning electron microscopy and energy dispersive X-ray spectroscopy

The morphology of GPA and GPA-Au were compared with a Hitachi SU5000 scanning electron microscope at 1-5 kV to lower charging effects. The composition of these aerogels was determined with energy-dispersive X-ray spectroscopy with a Hitachi TM3030 microscope possessing Oxford Instruments Swift ID.

Raman spectroscopy

Raman Spectrometer (Renishaw inVia Qontor) was calibrated with Silicon, characterised by a Raman peak at 520 cm−1 [57]. GPA, GPA-Au5nm or GPA-Au20nm were then placed into the chamber and Raman spectra were obtained at both 514 nm and 633 nm wavelengths. The 633 nm spectra were recorded with × 50 objective, 5% laser power (719 µW), 10 s acquisition time and 10 accumulations and the 514 nm spectra were recorded with a × 20 objective, 5% laser power (1.03 mW), 60 s acquisition time and 3 accumulations. The spectra were processed with Python, subtracting the baseline and optimising the peak fitting.

Conductivity measurements

The conductivity of the GPAs was measured with a Keithley 617 Electrometer. 100 µL of GPH were dried on 10 × 10 mm glass substrates with conductive metallic tape (RS Pro) spanning 25 mm at each end, allowing the current to pass through 5 mm of the resulting GPA. An electrical connection between the GPA and conductive tape was ensured with silver conductive lacquer (RS Pro). To minimise current leakage, triaxial cables were used and the set-up was placed within a grounded outer electromagnetic interference shielding enclosure.

Acknowledgements

We acknowledge the BAE Systems (BS/1464085) and the EPSRC Centre for Doctoral Training in Formulation Engineering. We also acknowledge funding by the EPSRC (EP/V029983/1 and EP/W004593/1) and the Wellcome Trust (174ISSFPP).

Author Contribution

K.S., Y.S. and P.G.O.: Conceptualization, K.S., Y.S., J.T., P.P., P.G.O.: Methodology, K.S., Y.S., P.G.O.: Data curation, All Authors: Writing- Original draft preparation. K.S., Y.S., H.W., P.G.O.: Visualization, Investigation. P.G.O.: Supervision. K.S., Y.S., P.G.O.: Validation. All Authors: Writing- Reviewing and Editing.

Data availability

Data is provided within the manuscript. Raw data of this study is available from the corresponding author upon request.

Declarations

Competing interests

The authors declare no competing interests.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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References

1. Urade AR Lahiri I Suresh KS Graphene properties, synthesis and applications: a review JOM 2023 75 3 614 630 10.1007/s11837-022-05505-8 36267692
Urade AR, Lahiri I, Suresh KS. Graphene properties, synthesis and applications: a review. JOM. 2023;75(3):614–30. 10.1007/s11837-022-05505-8.36267692
2. Passaretti P Graphene oxide and biomolecules for the production of functional 3D graphene-based materials Front Mol Biosci 2022 10.3389/fmolb.2022.774097 35372519
Passaretti P. Graphene oxide and biomolecules for the production of functional 3D graphene-based materials. Front Mol Biosci. 2022. 10.3389/fmolb.2022.774097.35372519
3. Aliyev E Filiz V Khan MM Lee YJ Abetz C Abetz V Structural characterization of graphene oxide: surface functional groups and fractionated oxidative debris Nanomaterials (Basel) 2019 10.3390/nano9081180 31426617
Aliyev E, Filiz V, Khan MM, Lee YJ, Abetz C, Abetz V. Structural characterization of graphene oxide: surface functional groups and fractionated oxidative debris. Nanomaterials (Basel). 2019. 10.3390/nano9081180.31426617
4. Azizi-Lalabadi M Jafari SM Bio-nanocomposites of graphene with biopolymers; fabrication, properties, and applications Adv Colloid Interface Sci 2021 292 102416 10.1016/j.cis.2021.102416 33872984
Azizi-Lalabadi M, Jafari SM. Bio-nanocomposites of graphene with biopolymers; fabrication, properties, and applications. Adv Colloid Interface Sci. 2021;292:102416. 10.1016/j.cis.2021.102416.33872984
5. Hu X Huang H Hu Y Lu X Qin Y Novel bio-based composite phase change materials with reduced graphene oxide-functionalized spent coffee grounds for efficient solar-to-thermal energy storage Solar Energy Mater Solar Cells 2021 219 110790 10.1016/j.solmat.2020.110790
Hu X, Huang H, Hu Y, Lu X, Qin Y. Novel bio-based composite phase change materials with reduced graphene oxide-functionalized spent coffee grounds for efficient solar-to-thermal energy storage. Solar Energy Mater Solar Cells. 2021;219:110790. 10.1016/j.solmat.2020.110790.
6. Rostamian M Introducing a bio sorbent for removal of methylene blue dye based on flexible poly(glycerol sebacate)/chitosan/graphene oxide ecofriendly nanocomposites Chemosphere 2022 289 133219 10.1016/j.chemosphere.2021.133219 34902387
Rostamian M, et al. Introducing a bio sorbent for removal of methylene blue dye based on flexible poly(glycerol sebacate)/chitosan/graphene oxide ecofriendly nanocomposites. Chemosphere. 2022;289:133219. 10.1016/j.chemosphere.2021.133219.34902387
7. Ghamkhari A Abbaspour-Ravasjani S Talebi M Hamishehkar H Hamblin MR Development of a graphene oxide-poly lactide nanocomposite as a Smart Drug Delivery System Int J Biol Macromol 2021 169 521 531 10.1016/j.ijbiomac.2020.12.084 33340628
Ghamkhari A, Abbaspour-Ravasjani S, Talebi M, Hamishehkar H, Hamblin MR. Development of a graphene oxide-poly lactide nanocomposite as a Smart Drug Delivery System. Int J Biol Macromol. 2021;169:521–31. 10.1016/j.ijbiomac.2020.12.084.33340628
8. Stokes K Sun Y Passaretti P White H Goldberg Oppenheimer P Optimisation of GraPhage13 macro-dispersibility via understanding the pH-dependent ionisation during self-assembly: towards the manufacture of graphene-based nanodevices Nanoscale 2023 10.1039/D3NR00778B 37519099
Stokes K, Sun Y, Passaretti P, White H, Goldberg Oppenheimer P. Optimisation of GraPhage13 macro-dispersibility via understanding the pH-dependent ionisation during self-assembly: towards the manufacture of graphene-based nanodevices. Nanoscale. 2023. 10.1039/D3NR00778B.37519099
9. Park IW Recent developments and prospects of M13- bacteriophage based piezoelectric energy harvesting devices Nanomaterials (Basel) 2020 10.3390/nano10010093 33379350
Park IW, et al. Recent developments and prospects of M13- bacteriophage based piezoelectric energy harvesting devices. Nanomaterials (Basel). 2020. 10.3390/nano10010093.33379350
10. Passaretti P Multifunctional graphene oxide-bacteriophage based porous three-dimensional micro-nanocomposites Nanoscale 2019 11 28 13318 13329 10.1039/C9NR03670A 31271408
Passaretti P, et al. Multifunctional graphene oxide-bacteriophage based porous three-dimensional micro-nanocomposites. Nanoscale. 2019;11(28):13318–29. 10.1039/C9NR03670A.31271408
11. Chabot V Higgins D Yu A Xiao X Chen Z Zhang J A review of graphene and graphene oxide sponge: material synthesis and applications to energy and the environment Energy Environ Sci 2014 7 5 1564 1596 10.1039/C3EE43385D
Chabot V, Higgins D, Yu A, Xiao X, Chen Z, Zhang J. A review of graphene and graphene oxide sponge: material synthesis and applications to energy and the environment. Energy Environ Sci. 2014;7(5):1564–96. 10.1039/C3EE43385D.
12. Orangi J Tetik H Parandoush P Kayali E Lin D Beidaghi M Conductive and highly compressible MXene aerogels with ordered microstructures as high-capacity electrodes for Li-ion capacitors Mater Today Adv 2021 9 100135 10.1016/j.mtadv.2021.100135
Orangi J, Tetik H, Parandoush P, Kayali E, Lin D, Beidaghi M. Conductive and highly compressible MXene aerogels with ordered microstructures as high-capacity electrodes for Li-ion capacitors. Mater Today Adv. 2021;9:100135. 10.1016/j.mtadv.2021.100135.
13. Pottathara YB Tiyyagura HR Ahmad Z Sadasivuni KK Graphene based aerogels: fundamentals and applications as supercapacitors J Energy Storage 2020 30 101549 10.1016/j.est.2020.101549
Pottathara YB, Tiyyagura HR, Ahmad Z, Sadasivuni KK. Graphene based aerogels: fundamentals and applications as supercapacitors. J Energy Storage. 2020;30:101549. 10.1016/j.est.2020.101549.
14. Wei S Qiu X An J Chen Z Zhang X Highly sensitive, flexible, green synthesized graphene/biomass aerogels for pressure sensing application Compos Sci Technol 2021 207 108730 10.1016/j.compscitech.2021.108730
Wei S, Qiu X, An J, Chen Z, Zhang X. Highly sensitive, flexible, green synthesized graphene/biomass aerogels for pressure sensing application. Compos Sci Technol. 2021;207:108730. 10.1016/j.compscitech.2021.108730.
15. Long C Supercapacitive brophene-graphene aerogel as elastic-electrochemical dielectric layer for sensitive pressure sensors J Colloid Interface Sci 2021 601 355 364 10.1016/j.jcis.2021.05.116 34087596
Long C, et al. Supercapacitive brophene-graphene aerogel as elastic-electrochemical dielectric layer for sensitive pressure sensors. J Colloid Interface Sci. 2021;601:355–64. 10.1016/j.jcis.2021.05.116.34087596
16. Bibi A Comparative study on the H2S gas-sensing properties of graphene aerogels synthesized through hydrothermal and chemical reduction J Taiwan Inst Chem Eng 2024 154 105155 10.1016/j.jtice.2023.105155
Bibi A, et al. Comparative study on the H2S gas-sensing properties of graphene aerogels synthesized through hydrothermal and chemical reduction. J Taiwan Inst Chem Eng. 2024;154:105155. 10.1016/j.jtice.2023.105155.
17. Li Q Ag-modified 3D reduced graphene oxide aerogel-based sensor with an embedded microheater for a fast response and high-sensitive detection of NO2 ACS Appl Mater Interfaces 2020 12 22 25243 25252 10.1021/acsami.9b22098 32391684
Li Q, et al. Ag-modified 3D reduced graphene oxide aerogel-based sensor with an embedded microheater for a fast response and high-sensitive detection of NO2. ACS Appl Mater Interfaces. 2020;12(22):25243–52. 10.1021/acsami.9b22098.32391684
18. dos Santos-Gómez L García JR Montes-Morán MA Menéndez JA García-Granda S Arenillas A Ultralight-weight graphene aerogels with extremely high electrical conductivity Small 2021 17 41 2103407 10.1002/smll.202103407
dos Santos-Gómez L, García JR, Montes-Morán MA, Menéndez JA, García-Granda S, Arenillas A. Ultralight-weight graphene aerogels with extremely high electrical conductivity. Small. 2021;17(41):2103407. 10.1002/smll.202103407.
19. Gupta S Joshi P Narayan J Electron mobility modulation in graphene oxide by controlling carbon melt lifetime Carbon 2020 170 327 337 10.1016/j.carbon.2020.07.073
Gupta S, Joshi P, Narayan J. Electron mobility modulation in graphene oxide by controlling carbon melt lifetime. Carbon. 2020;170:327–37. 10.1016/j.carbon.2020.07.073.
20. Sun Y Nanomechanics of graphene oxide-bacteriophage based self-assembled porous composites Sci Rep 2020 10 1 15618 10.1038/s41598-020-72372-1 32973218
Sun Y, et al. Nanomechanics of graphene oxide-bacteriophage based self-assembled porous composites. Sci Rep. 2020;10(1):15618. 10.1038/s41598-020-72372-1.32973218
21. Li GJ Zhang XH Kawi S Relationships between sensitivity, catalytic activity, and surface areas of SnO2 gas sensors Sensors Actuators B Chem 1999 60 1 64 70 10.1016/S0925-4005(99)00245-2
Li GJ, Zhang XH, Kawi S. Relationships between sensitivity, catalytic activity, and surface areas of SnO2 gas sensors. Sensors Actuators B Chem. 1999;60(1):64–70. 10.1016/S0925-4005(99)00245-2.
22. Raju P Li Q Review—Semiconductor materials and devices for gas sensors J Electrochem Soc 2022 169 5 057518 10.1149/1945-7111/ac6e0a
Raju P, Li Q. Review—Semiconductor materials and devices for gas sensors. J Electrochem Soc. 2022;169(5):057518. 10.1149/1945-7111/ac6e0a.
23. Srinivasa Varaprasad H Sridevi PV Satya Anuradha M Optical, morphological, electrical properties of ZnO–TiO2–SnO2/CeO2 semiconducting ternary nanocomposite Adv Powder Technol 2021 32 5 1472 1480 10.1016/j.apt.2021.02.042
Srinivasa Varaprasad H, Sridevi PV, Satya Anuradha M. Optical, morphological, electrical properties of ZnO–TiO2–SnO2/CeO2 semiconducting ternary nanocomposite. Adv Powder Technol. 2021;32(5):1472–80. 10.1016/j.apt.2021.02.042.
24. Gou Q Highly selective Pb(II) adsorption by DTPA-functionalized graphene oxide/carboxymethyl cellulose aerogel Langmuir 2024 40 15 8002 8014 10.1021/acs.langmuir.3c03954 38566445
Gou Q, et al. Highly selective Pb(II) adsorption by DTPA-functionalized graphene oxide/carboxymethyl cellulose aerogel. Langmuir. 2024;40(15):8002–14. 10.1021/acs.langmuir.3c03954.38566445
25. Pan H Low S Weerasuriya N Wang B Shon Y-S Morphological transformation of gold nanoparticles on graphene oxide: effects of capping ligands and surface interactions Nano Convergence 2019 6 1 2 10.1186/s40580-018-0171-0 30617903
Pan H, Low S, Weerasuriya N, Wang B, Shon Y-S. Morphological transformation of gold nanoparticles on graphene oxide: effects of capping ligands and surface interactions. Nano Convergence. 2019;6(1):2. 10.1186/s40580-018-0171-0.30617903
26. Mitsubayashi K Toma K Iitani K Arakawa T Gas-phase biosensors: a review Sensors Actuators B: Chem 2022 367 132053 10.1016/j.snb.2022.132053
Mitsubayashi K, Toma K, Iitani K, Arakawa T. Gas-phase biosensors: a review. Sensors Actuators B: Chem. 2022;367:132053. 10.1016/j.snb.2022.132053.
27. Panigrahi PK Chandu B Puvvada N Recent advances in nanostructured materials for application as gas sensors ACS Omega 2024 9 3 3092 3122 10.1021/acsomega.3c06533 38284032
Panigrahi PK, Chandu B, Puvvada N. Recent advances in nanostructured materials for application as gas sensors. ACS Omega. 2024;9(3):3092–122. 10.1021/acsomega.3c06533.38284032
28. Mittal R Kumar A Awasthi SK Practical scale up synthesis of carboxylic acids and their bioisosteres 5-substituted-1H-tetrazoles catalyzed by a graphene oxide-based solid acid carbocatalyst RSC Adv 2021 11 19 11166 11176 10.1039/D1RA01053K 35423636
Mittal R, Kumar A, Awasthi SK. Practical scale up synthesis of carboxylic acids and their bioisosteres 5-substituted-1H-tetrazoles catalyzed by a graphene oxide-based solid acid carbocatalyst. RSC Adv. 2021;11(19):11166–76. 10.1039/D1RA01053K.35423636
29. Xia X Carboxyl functionalized gold nanoparticles in situ grown on reduced graphene oxide for micro-gravimetric ammonia sensing Sensors Actuators B: Chem 2014 202 846 853 10.1016/j.snb.2014.06.029
Xia X, et al. Carboxyl functionalized gold nanoparticles in situ grown on reduced graphene oxide for micro-gravimetric ammonia sensing. Sensors Actuators B: Chem. 2014;202:846–53. 10.1016/j.snb.2014.06.029.
30. Stokes K Sun Y Zhang H Passaretti P White H Goldberg Oppeneheimer P Thermonanomechanics of graphene oxide-M13 bacteriophage nanocomposites -towards graphene-based nanodevices Carbon Trends 2024 15 100343 10.1016/j.cartre.2024.100343
Stokes K, Sun Y, Zhang H, Passaretti P, White H, Goldberg Oppeneheimer P. Thermonanomechanics of graphene oxide-M13 bacteriophage nanocomposites -towards graphene-based nanodevices. Carbon Trends. 2024;15:100343. 10.1016/j.cartre.2024.100343.
31. Ferrari AC Raman spectroscopy of graphene and graphite: disorder, electron–phonon coupling, doping and nonadiabatic effects Solid State Commun 2007 143 1 47 57 10.1016/j.ssc.2007.03.052
Ferrari AC. Raman spectroscopy of graphene and graphite: disorder, electron–phonon coupling, doping and nonadiabatic effects. Solid State Commun. 2007;143(1):47–57. 10.1016/j.ssc.2007.03.052.
32. Pardanaud C, Cartry G, Lajaunie L, Arenal R, Buijnsters JG. Investigating the possible origin of Raman bands in defective sp 2/sp 3 carbons below 900 cm−1: phonon density of states or double resonance mechanism at play? C. 2019;5(4), 79. 10.3390/c5040079
33. Ferrari AC Robertson J Ferrari AC Robertson J Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond Philos Trans R Soc Lond Ser A: Math Phys Eng Sci 2004 362 1824 2477 2512 10.1098/rsta.2004.1452
Ferrari AC, Robertson J, Ferrari AC, Robertson J. Raman spectroscopy of amorphous, nanostructured, diamond-like carbon, and nanodiamond. Philos Trans R Soc Lond Ser A: Math Phys Eng Sci. 2004;362(1824):2477–512. 10.1098/rsta.2004.1452.
34. Tagliaferro A Rovere M Padovano E Bartoli M Giorcelli M Introducing the novel mixed Gaussian–Lorentzian lineshape in the analysis of the Raman signal of biochar Nanomaterials 2020 10 9 1748 10.3390/nano10091748 32899319
Tagliaferro A, Rovere M, Padovano E, Bartoli M, Giorcelli M. Introducing the novel mixed Gaussian–Lorentzian lineshape in the analysis of the Raman signal of biochar. Nanomaterials. 2020;10(9):1748. 10.3390/nano10091748.32899319
35. Ferrari AC Robertson J Interpretation of Raman spectra of disordered and amorphous carbon Phys Rev B 2000 61 20 14095 14107 10.1103/PhysRevB.61.14095
Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B. 2000;61(20):14095–107. 10.1103/PhysRevB.61.14095.
36. Ferrari AC Basko DM Raman spectroscopy as a versatile tool for studying the properties of graphene Nat Nanotechnol 2013 8 4 235 246 10.1038/nnano.2013.46 23552117
Ferrari AC, Basko DM. Raman spectroscopy as a versatile tool for studying the properties of graphene. Nat Nanotechnol. 2013;8(4):235–46. 10.1038/nnano.2013.46.23552117
37. Wu J-B Lin M-L Cong X Liu H-N Tan P-H Raman spectroscopy of graphene-based materials and its applications in related devices Chem Soc Rev 2018 47 5 1822 1873 10.1039/C6CS00915H 29368764
Wu J-B, Lin M-L, Cong X, Liu H-N, Tan P-H. Raman spectroscopy of graphene-based materials and its applications in related devices. Chem Soc Rev. 2018;47(5):1822–73. 10.1039/C6CS00915H.29368764
38. Kar R Maiti N Observation of D band splitting in vertically aligned graphene nanowalls and their evolution with laser power during Raman spectroscopy J Nanopart Res 2021 23 1 4 10.1007/s11051-021-05147-y
Kar R, Maiti N. Observation of D band splitting in vertically aligned graphene nanowalls and their evolution with laser power during Raman spectroscopy. J Nanopart Res. 2021;23(1):4. 10.1007/s11051-021-05147-y.
39. Lee JH Cho HY Choi HK Lee JY Choi JW Application of gold nanoparticle to plasmonic biosensors Int J Mol Sci 2018 10.3390/ijms19072021 30586948
Lee JH, Cho HY, Choi HK, Lee JY, Choi JW. Application of gold nanoparticle to plasmonic biosensors. Int J Mol Sci. 2018. 10.3390/ijms19072021.30586948
40. Link S El-Sayed MA Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles J Phys Chem B 1999 103 21 4212 4217 10.1021/jp984796o
Link S, El-Sayed MA. Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles. J Phys Chem B. 1999;103(21):4212–7. 10.1021/jp984796o.
41. Palaniselvam T Aiyappa HB Kurungot S An efficient oxygen reduction electrocatalyst from graphene by simultaneously generating pores and nitrogen doped active sites J Mater Chem 2012 22 45 23799 23805 10.1039/C2JM35128E
Palaniselvam T, Aiyappa HB, Kurungot S. An efficient oxygen reduction electrocatalyst from graphene by simultaneously generating pores and nitrogen doped active sites. J Mater Chem. 2012;22(45):23799–805. 10.1039/C2JM35128E.
42. Armas LEG Effect of gold nanoparticles and unwanted residues on Raman spectra of graphene sheets Braz J Phys 2018 48 5 477 484 10.1007/s13538-018-0596-2
Armas LEG, et al. Effect of gold nanoparticles and unwanted residues on Raman spectra of graphene sheets. Braz J Phys. 2018;48(5):477–84. 10.1007/s13538-018-0596-2.
43. Nancy P Nair AK Antoine R Thomas S Kalarikkal N In situ decoration of gold nanoparticles on graphene oxide via nanosecond laser ablation for remarkable chemical sensing and catalysis Nanomaterials 2019 9 9 1201 10.3390/nano9091201 31455035
Nancy P, Nair AK, Antoine R, Thomas S, Kalarikkal N. In situ decoration of gold nanoparticles on graphene oxide via nanosecond laser ablation for remarkable chemical sensing and catalysis. Nanomaterials. 2019;9(9):1201. 10.3390/nano9091201.31455035
44. BalajiMohan V Stamm M Bhattacharyya D Liu D Jayaraman K Improvements in electronic structure and properties of graphene derivatives Adv Mater Lett 2016 7 6 421 429 10.5185/amlett.2016.6123
BalajiMohan V, Stamm M, Bhattacharyya D, Liu D, Jayaraman K. Improvements in electronic structure and properties of graphene derivatives. Adv Mater Lett. 2016;7(6):421–9. 10.5185/amlett.2016.6123.
45. Zainab S Azeem M Awan SU Rizwan S Iqbal N Rashid J Optimization of bandgap reduction in 2-dimensional GO nanosheets and nanocomposites of GO/iron-oxide for electronic device applications Sci Rep 2023 13 1 6954 10.1038/s41598-023-33200-4 37117234
Zainab S, Azeem M, Awan SU, Rizwan S, Iqbal N, Rashid J. Optimization of bandgap reduction in 2-dimensional GO nanosheets and nanocomposites of GO/iron-oxide for electronic device applications. Sci Rep. 2023;13(1):6954. 10.1038/s41598-023-33200-4.37117234
46. Wang X High electrical conductivity and carrier mobility in oCVD PEDOT thin films by engineered crystallization and acid treatment Sci Adv 2018 4 9 eaat5780 10.1126/sciadv.aat5780 30225366
Wang X, et al. High electrical conductivity and carrier mobility in oCVD PEDOT thin films by engineered crystallization and acid treatment. Sci Adv. 2018;4(9):eaat5780. 10.1126/sciadv.aat5780.30225366
47. Perumbilavil S Sankar P Thankamani PR Philip R White light Z-scan measurements of ultrafast optical nonlinearity in reduced graphene oxide nanosheets in the 400–700 nm region Appl Phys Lett 2015 107 051104 10.1063/1.4928124
Perumbilavil S, Sankar P, Thankamani PR, Philip R. White light Z-scan measurements of ultrafast optical nonlinearity in reduced graphene oxide nanosheets in the 400–700 nm region. Appl Phys Lett. 2015;107:051104. 10.1063/1.4928124.
48. Abid, Sehrawat P, Islam SS, Mishra P, Ahmad S. Reduced graphene oxide (rGO) based wideband optical sensor and the role of temperature, defect states and quantum efficiency. Sci Rep. 2018;8(1), 3537. 10.1038/s41598-018-21686-2.
49. Jin Y Zheng Y Podkolzin SG Lee W Band gap of reduced graphene oxide tuned by controlling functional groups J Mater Chem C 2020 8 14 4885 4894 10.1039/C9TC07063J
Jin Y, Zheng Y, Podkolzin SG, Lee W. Band gap of reduced graphene oxide tuned by controlling functional groups. J Mater Chem C. 2020;8(14):4885–94. 10.1039/C9TC07063J.
50. Cho H Bae G Hong BH Engineering functionalization and properties of graphene quantum dots (GQDs) with controllable synthesis for energy and display applications Nanoscale 2024 16 7 3347 3378 10.1039/D3NR05842E 38288500
Cho H, Bae G, Hong BH. Engineering functionalization and properties of graphene quantum dots (GQDs) with controllable synthesis for energy and display applications. Nanoscale. 2024;16(7):3347–78. 10.1039/D3NR05842E.38288500
51. Pereira NGA Gonzaléz MEL Queiroz AAAd Oliveira AF Tavares Wanderley Neto E Higher electrical conductivity of functionalized graphene oxide doped with silver and copper (II) ions Energies 2023 16 20 7019 10.3390/en16207019
Pereira NGA, Gonzaléz MEL, Queiroz AAAd, Oliveira AF, Tavares Wanderley Neto E. Higher electrical conductivity of functionalized graphene oxide doped with silver and copper (II) ions. Energies. 2023;16(20):7019. 10.3390/en16207019.
52. Ngidi NPD Ollengo MA Nyamori VO Tuning the properties of boron-doped reduced graphene oxide by altering the boron content New J Chem 2020 44 39 16864 16876 10.1039/D0NJ03909H
Ngidi NPD, Ollengo MA, Nyamori VO. Tuning the properties of boron-doped reduced graphene oxide by altering the boron content. New J Chem. 2020;44(39):16864–76. 10.1039/D0NJ03909H.
53. Idisi DO Benecha EM Moloi SJ Ray SC Effects of gold nanoparticles (Au-NPs) on the electrical properties of reduced graphene oxide: an experimental and DFT study J Mater Res 2022 37 5 1037 1046 10.1557/s43578-022-00514-4
Idisi DO, Benecha EM, Moloi SJ, Ray SC. Effects of gold nanoparticles (Au-NPs) on the electrical properties of reduced graphene oxide: an experimental and DFT study. J Mater Res. 2022;37(5):1037–46. 10.1557/s43578-022-00514-4.
54. Laroui H Nanomedicine in GI Am J Physiol Gastrointest Liver Physiol 2010 300 G371 G383 10.1152/ajpgi.00466.2010 21148398
Laroui H, et al. Nanomedicine in GI. Am J Physiol Gastrointest Liver Physiol. 2010;300:G371–83. 10.1152/ajpgi.00466.2010.21148398
55. Wang Y Enhanced dispersion stability of gold nanoparticles by the physisorption of cyclic poly(ethylene glycol) Nat Commun 2020 11 1 6089 10.1038/s41467-020-19947-8 33257670
Wang Y, et al. Enhanced dispersion stability of gold nanoparticles by the physisorption of cyclic poly(ethylene glycol). Nat Commun. 2020;11(1):6089. 10.1038/s41467-020-19947-8.33257670
56. Morag O Sgourakis NG Abramov G Goldbourt A Ghose R Filamentous bacteriophage viruses: preparation, magic-angle spinning solid-state NMR experiments, and structure determination Protein NMR: methods and protocols 2018 New York, NY Springer 67 97
Morag O, Sgourakis NG, Abramov G, Goldbourt A. Filamentous bacteriophage viruses: preparation, magic-angle spinning solid-state NMR experiments, and structure determination. In: Ghose R, editor. Protein NMR: methods and protocols. New York, NY: Springer; 2018. p. 67–97.
57. Uchinokura K Sekine T Matsuura E Raman scattering by silicon Solid State Commun 1972 11 1 47 49 10.1016/0038-1098(72)91127-1
Uchinokura K, Sekine T, Matsuura E. Raman scattering by silicon. Solid State Commun. 1972;11(1):47–9. 10.1016/0038-1098(72)91127-1.
