# Sustainable Nanocarbons via Biocatalysis of C-F Bond Cleavage by Electroactive Bacteria

Sarra Knani, Riad Ahmed Boughezal, Baptiste Cibron, Frédéric Barrière, Corinne Lagrost, Muriel Escadeillas, Ludivine Rault, Maida Costa de Oliveira, Paula E Colavita, James A Behan

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Sarra Knani, Riad Ahmed Boughezal, Baptiste Cibron, Frédéric Barrière, Corinne Lagrost, et al. Sustainable Nanocarbons via Biocatalysis of C-F Bond Cleavage by Electroactive Bacteria. Carbon, 2026, pp.121053. 10.1016/j.carbon.2025.121053. hal-05360526

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# Sustainable nanocarbons via biocatalysis of C–F bond cleavage by electroactive bacteria

Sarra Knani<sup>a</sup>, Riad Ahmed Boughezal<sup>a</sup>, Baptiste Cibron<sup>a</sup>, Frédéric Barrière<sup>a, b, c</sup>, Corinne Lagrost<sup>a, b, d</sup>, Muriel Escadeillas<sup>a</sup>, Ludivine Rault<sup>b</sup>, Maida Costa de Oliveira<sup>c, d</sup>, Paula E. Colavita<sup>c, d</sup>, James A. Behan<sup>a, *, d</sup>

## ARTICLE INFO

### Keywords:

Electroactive bacteria

Biocatalysis

Nanocarbons

Bioexfoliation

Fluorographene

## ABSTRACT

Fluorographite is a promising starting material for the synthesis of graphene and graphene oxides via top-down exfoliation routes. C–F bonds in fluorographite can be chemically substituted to form graphene oxides with tuned surface chemistry. However, most defluorination routes reported proceed via fluorographene exfoliation from fluorographite followed by refluxing in organic solvents. Herein a one-pot, biogenic and aqueous route to graphene oxide nanomaterials directly from fluorographite with no pre-exfoliation step is demonstrated. Fluorographite incubation with the electroactive bacterial species *Geobacter sulfurreducens* yields water-dispersible graphene oxide quantum dots and partially-defluorinated graphene oxides as the main nanomaterial products in a single step. Biocatalysis at ambient temperature and pH proceeds through direct surface contact with bacteria through C–F bond cleavage, with fluorographite serving as the sole electron acceptor for *exo*-electrogenic respiration. This bioexfoliation strategy presents a sustainable and green synthetic route to functionalised carbon nanomaterials with tuneable size and surface properties under ambient conditions.

## 1. Introduction

Fluorinated nanocarbons including fluorographenes have emerged as interesting materials for a variety of applications ranging from supercapacitors [1] and battery materials [2,3] to perovskite solar cells [4]. Historically the fluorination of graphite to form graphite fluoride (“fluorographite”) produced excellent solid lubricant materials [5]. Nowadays fluorographite and its exfoliated derivatives are of great interest as composite materials in energy storage and conversion, catalysis and biosensing [6,7]. Though it was initially described as a 2-D analogue of polytetrafluoroethylene, it is now known that fluorographene is a reactive material suitable for the preparation of modified graphene materials [6,8–10]. This followed the first exfoliation of fluorographene from the bulk fluorographite in 2010 [11,12], along with its conversion to graphene under relatively mild conditions. The electrophilic carbons in fluorographene are susceptible to reductive defluorination and/or nucleophilic substitution [13], allowing for a remarkable variety of surface-modified or doped graphene materials to be prepared [6,9,

14–16]. This top-down approach usually involves ultrasonication starting from graphite fluoride to yield the fluorographene, followed by reflux in organic solvents to eliminate fluorine and replace it with other moieties (Fig. 1 a) [6,8,9,14,17–19]. Aqueous media routes to modified nanocarbons with fewer processing steps are therefore desirable, but are limited by the low water-solubility of the extremely hydrophobic fluorographite precursor.

Electroactive bacteria, i.e., bacteria that can transfer electrons extracellularly, are capable of exchanging electrons with solid materials including electrodes and oxides of metals such as Fe(III) [20]. These redox processes are commonly anodic, i.e. involving the transfer of intracellular electrons derived from organic matter to an external electron acceptor, as found in iron-rich soils and the bioanodes of microbial fuel cell systems [21–24]. However it is also possible to develop cathodic systems, where electrons are transferred from extracellular solids to the cell interior. Examples of this include Fe(II) oxidizing phototrophic bacteria, which can use Fe(II)-rich minerals as the electron source for anoxygenic photosynthesis [25,26]. When the capacity for extracellular

<footnote><sup>a</sup> Université de Rennes, CNRS, Institut des Sciences Chimiques de Rennes, UMR 6226, F-35000, Rennes, France</footnote>

<footnote><sup>b</sup> Université de Rennes, CNRS, ScanMAT, UAR 2025, F-35000, Rennes, France</footnote>

<footnote><sup>c</sup> School of Chemistry, CRANN and AMBER Research Centres, Trinity College Dublin, College Green, Dublin 2, Ireland</footnote>

<footnote>* Corresponding author.
E-mail address: james.behan@univ-rennes.fr (J.A. Behan).</footnote>
electron transfer is applied to the synthesis of GO derivatives, the so-called 'bio-exfoliation' of GO usually involves the oxidation of dispersed graphite or graphene materials [27–29] where the starting material serves as the electron donor. For Geobacter sulfurreducens and related species electrons derived from the respiratory chain at the inner membrane are transferred to external solids via c-type cytochrome proteins which are localized at the outer membrane and secreted in the form of conductive protein nanowires which can carry electrons across long distances outside of the cell [21,30,31]. Another species of electroactive bacteria, Shewanella oneidensis, has been shown to reduce nanocarbons such as graphene oxide (GO) via its own extracellular respiratory machinery [32,33]. In both of these cases the extracellular solid serves as the terminal electron acceptor. Biological redox pathways may therefore serve as versatile routes to drive nanomaterial synthesis under mild conditions, including the synthesis of diverse nanocarbons with varied surface chemistry.

We hypothesized (Fig. 1 b) that the relatively labile C–F bonds in graphitic fluorides could in principle serve as the electron acceptor for extracellular respiration by electroactive bacteria and hence potentially be cleaved via biocatalysis under mild, biocompatible conditions. Such a direct “bioexfoliation” route of bulk fluorographite to nanocarbons is unprecedented and would allow for the versatile surface chemistry of graphitic fluorides to be exploited to prepare GO and heteroatom-doped GO derivatives tailored to catalytic, imaging or biosensing applications under more sustainable conditions than those presently employed.

In this work we present the simultaneous bioexfoliation and defluorination of bulk fluorographite to GO and partially-fluorinated GO (fGO) based nanocarbons biocatalysed by the electroactive bacterium G. sulfurreducens. The process operates at mild temperatures (35 °C) and at neutral pH, with no need for fluorographene exfoliation through ultrasonication methods. We find superior defluorination efficiencies

with this biological method compared to previously reported abiotic routes applying physical exfoliation of fluorographite followed by high temperature reflux in organic solvents. A range of nanocarbons with tunable size, surface chemistry and fluorination degree are produced under aqueous conditions in a one pot incubation with only the bacteria, an organic electron donor, and the fluorographite as the presumed terminal electron acceptor for extracellular respiration. We show that the process is metabolically-driven, demonstrating a previously unknown capacity for these bacteria to biocatalytically cleave C–F. We propose a direct electron transfer mechanism from the bacteria to the fluorographite by outer-membrane cytochrome proteins at the bacteria surface. Reductive cleavage leads to both defluorination and subsequent delamination of the graphitic starting material to produce valuable nanocarbon products, including ultrasmall graphene oxide quantum dots (GQDs), fGO nanoparticles and fluorographene sheets. Our results show how a biocatalytic approach can streamline the synthesis of nanomaterials with tailored physicochemical properties under ambient conditions.

## 2. Materials and methods

### 2.1. Chemicals and materials

Fluorographite (polymer, >61 wt % F, Sigma Aldrich), sodium acetate (≥99 %), riboflavin (Vitamin B2, ≥99 %), sodium fumarate dibasic (≥99 %), ammonium chloride (≥99.5 %), potassium chloride (molecular biology grade (≥99.0 %), NaH₂PO₄ (BioReagent, ≥98 %), Na₂HPO₄ (ACS. ≥99.0 %), Na₂SO₄ (≥99.0 %), MgCl₂·6H₂O (BioReagent) and CaCl₂·2H₂O (BioXtra, ≥99.0 %) and cytochrome c from equine heart were purchased from Sigma Aldrich. BBL™ Yeast extract was purchased from BD Biosciences. All chemicals were used as-received without

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** a) Illustrating a typical route to prepare GO nanocarbons from graphite fluoride precursors using ultrasonication to form fluorographene followed by organic solvent treatments to cleave C–F bonds. b) We present a one-pot biological route using the metabolic activity of electroactive bacteria to simultaneously bioexfoliate and cleave C–F bonds in fluorographite, forming a range of GO and fluorinated-GO nanocarbon products under ambient conditions in aqueous media.</figcaption></figure>
further purification.

## 2.2. Culturing of *G. sulfurreducens* and incubation with fluorographite

*G. sulfurreducens* subsp. *sulfurreducens* strain PCA were anaerobically cultured in a modified acetate-fumarate media (the full media composition is given in **Supplementary Note 1** in the **Supplementary Information**) from cryostocks stored in 20 % glycerol at -80 °C. Cultures were grown until the stationary phase, at which point the optical density at 600 nm (OD₆₀₀) was ca. 0.2 as measured using 1 mL aliquots in quartz cuvettes via a dual-beam JASCO V-630 BIO spectrophotometer. The measured OD₆₀₀ corresponds to a cell dry weight biomass of 0.66 g/L OD₆₀₀, consistent with previous reports for *G. sulfurreducens* [34]. Cells were harvested by centrifugation at 5000×g for 10 min to pellet the bacteria followed by 3 washes with the desired incubation buffer. Minimal media contained only buffer (PBS or bicarbonate, pH 6.8-7) and 10 mM acetate and *G. sulfurreducens* at an OD₆₀₀ of 0.4 in the presence of fluorographite (5-10 g/L). This media was also supplemented with 10 µM riboflavin as a redox shuttle in certain experiments as described in the main text. Biological replicates of the entire experiment were carried out three times starting from fresh cryostocks. All samples were incubated in darkness a thermostatic cabinet at 35 °C without agitation. Samples were regularly taken aseptically using an anoxic syringe to check the pH and optical density. Bacterial viability in the presence of flurographite was verified by adding 5 g/L of the powder to acetate-fumarate media, which did not affect the apparent OD₆₀₀ at ca. 0.2 after 3 days of growth. After the incubation the solution was centrifuged twice at 5000×g for 20 min to pellet bacteria and insoluble precipitated material, followed by treatment at 80 °C with 2 % SDS and filter purification using 100 kDa MW cutoff Amicon® Ultra 15 mL filters to remove the incubation buffer and proteins. Experiments to demonstrate the presence of cytochrome proteins produced by the bacteria skipped the SDS treatment and removed the incubation buffer using 10 kDA MW cutoff Amicon® Ultra 15 mL filters while retaining the proteins and GQDs prior to characterization. The precipitates were washed 5 times with ultrapure water to remove incubation buffer and adherent biofilm prior to characterisation. Additional centrifugations at 20000×g for 3 cycles were applied to the colloids for size-selection of the resulting nanoparticles before TEM analysis. In all cases the final GQDs were concentrated to a volume of 5 mL in Millipore water using Amicon® filters and stored at room temperature prior to characterisation. The overall yield of bioexfoliated products, determined after oven-drying the products and unreacted starting material, was found to be 35 ± 5 % based on 3 replicates after 9 days of incubation. This yield is comparable to previous works using ultrasonication, solvothermal intercalation [35] and ball-milling approaches [36] to prepare fluorographene.

## 2.3. Characterisation of fluorographite

### 2.3.1. UV-visible spectroscopy characterisation

UV-Visible spectroscopy was carried out using a dual-beam JASCO V-630 BIO spectrophotometer using quartz cuvettes. Spectra were taken of purified GQDs in ultrapure water or directly in the growth media as described in the text. Pristine fluorographite was found to be nearly insoluble in water and was suspended in a 10 % ethanol/water solution. All measurements are baseline-corrected using the dispersion medium as a blank.

### 2.3.2. Fluorescence spectroscopy characterisation

Fluorescence spectra were recorded on a Horiba Fluoromax® FM4 ET THERMO fluorimeter. GQDs were diluted in ultrapure water or anhydrous ethanol as described in the text. Emission spectra were recorded with an interval of 1 nm and a 5 nm entrance/exit slit band-pass. Fluorescence intensities are presented after applying radiometric correction factors and normalised to the reference intensity to account for light source fluctuations.

### 2.3.3. X-ray photoelectron spectroscopy characterisation

Samples for XPS analysis were prepared by depositing powder (0.05g) in the case of pristine fluorographite or precipitated material or by drop-casting purified GQD colloid solutions after dilution in anhydrous ethanol and drying in air. p-Doped Si wafers (1-5 Ω cm resistivity, Siltronix) were used as substrates. XPS data were obtained using a NEXSA G2 (ThermoFisher Scientific) spectrometer using an Al Kα X-ray source working at 1486.6 eV with a spot size of 200 µm². Survey spectra (0-1000 eV) were acquired with an analyzer pass energy of 200 eV (1 eV/step, dwell time 10 ms); high-resolution spectra used a pass energy of 50 eV (0.1 eV/step, dwell time 50-300 ms). Charge compensation using a flood gun (P = 4 × 10⁻⁷ mbar) was applied to all powder preparations. The binding energies were corrected to the position of the C 1s spectral component at 284.8 eV assigned to C-C and C-H species. Spectra were peak-fitted using commercial software (CasaXPS Version 2.3.25 PR1.0). Deconvolutions were performed using U 2 Tougaard background subtraction in all cases. Peak areas used in quantitative analysis were normalised by relative sensitivity factors provided by the manufacturer for atomic percentage determination (C 1s = 1, O 1s = 2.88, N 1s = 1.68, F 1s = 4.12).

### 2.3.4. Fourier transform infrared (FTIR) spectroscopy

FTIR measurements were performed using a JASCO FT/IR-4600 system. Dried power samples of flurographite and bioexfoliation products were pressed into KBr pellets and analysed versus a baseline of pure KBr. For all samples 35 scans were collected in the range of 4000 to 500 cm⁻¹.

### 2.3.5. Raman spectroscopy characterisation

Raman measurements were performed on drop-cast powders or GQD solutions dispersed in anhydrous ethanol on Si wafers and drying at room temperature. Measurements were carried out on a LabRAM HR Evolution from Horiba Scientific using a × 100 objective with a 523 nm laser with a 10s accumulation time at 1 % laser intensity (0.15 mW power). The resulting spectra were deconvoluted using CasaXPS Version 2.3.25 PR1.0.

### 2.3.6. Transmission electron microscopy imaging

TEM images were acquired using a JEOL JEM 2100 HR transmission electron microscope equipped with a LaB₆ electron source with an acceleration voltage of 200 kV. A CCD Gatan Orius SC200D camera was used for conventional imaging while a CCD Gatan UltraScan 1000XP camera was used for high-resolution images. Samples of purified GQDs were diluted in anhydrous ethanol and deposited by drop-casting on copper grids coated with carbon/formvar films followed by air drying.

### 2.3.7. Scanning electron microscopy imaging

SEM images were acquired using a JEOL JSM 7100 F electron microscope. Samples were prepared by drop-casting powders or colloid solution directly on p-doped Si wafers and rinsing 3 times with ultrapure water before drying in air followed by critical point drying and sputter-coating with gold prior to imaging.

### 2.3.8. Atomic force microscopy imaging

AFM was performed on drop-cast samples deposited on Si wafers. Measurements were carried out in semi-contact mode using an Ntegra NT-MDT AFM using NSG10 tips (NT-MDT) with a resonant frequency of 47-150 kHz and a force constant of 37.6 N/m. The resulting images were analysed using open-source software (Gwyddion® 2.65) using the following data processing operations: *level data by mean plane subtraction, shift mean value to zero and correct horizontal scars (strokes)*.

### 2.3.9. Fluoride ion detection

After incubation with electroactive bacteria the spent medium was crudely purified via centrifugation to remove large particles and bacterial cells prior to measurement with a F 800 fluoride ion sensitive
electrode (ISE, supplied by WTW). The ISE was first calibrated with stock solutions of NaF with Total ionic strength adjustment buffer (TISAB II solution, Thermofisher®) from 10⁻⁵-10⁻³ M concentrations. (TISAB II, Thermofisher®) was added 1:1 to samples to allow comparability with standards.

### 3. Results and discussion

To develop our method, we incubated fluorographite with a formula (CFₓ)ₙ, x~1.1 (Fig. 2 a) with G. sulfurreducens in a minimal medium consisting of a buffer (PBS or bicarbonate buffer, pH 7) supplemented with 10 mM acetate as the sole electron donor and fluorographite (5–10 mg/mL) as the sole electron acceptor. 10 µM riboflavin was added as a redox shuttle [37], though this was not found to be essential for bioexfoliation. Fig. 2 a) shows the initial photograph directly after the reaction tubes were prepared. The white fluorographite powder is extremely hydrophobic and less dense than the liquid phase and floats at the liquid/gas interface. After 24h of incubation we identified regions of red biofilm formation characteristic of G. sulfurreducens localised at the meniscus in contact with the fluorographite (Fig. 2 a). A noticeable darkening of the white fluorographite at the interface is also evident along with a small amount of precipitation at the bottom of the tube. After 96 h of incubation the floating powder is noticeably grey in color, with a black layer of material forming at the surface of the liquid. No such changes were observed for the control tubes (Fig. 2 a and Fig. S1). The tubes were incubated for a further 6 d prior to the purification of the dispersion. At this point the solution had changed from the red color given by the G. sulfurreducens to a black-brown color consistent with colloidal nanocarbon dispersions.

#### 3.1. Characterisation of biogenic GO derivatives produced from fluorographite

After incubation the reaction tubes contained a mix of colloidal dispersions of particles, precipitates at the bottom of the tubes and unreacted fluorographite (a representative example is shown in Fig. 2 b) with our proposed bioexfoliation mechanism in Fig. 2 c). The unreacted starting material was separated from the solution by pipetting and washed to remove salts and biomass prior to analysis. The morphology of fluorographite consists of heterogeneously distributed fragments ranging from hundreds of nm to microns in size based on SEM images (Fig. 3 a and Figure S2 a). Energy dispersive X-ray (EDS) mapping of the pristine material confirms the given C:F ratio of 1:1.1 (Fig. S3). After removing the starting material, the colloid and precipitates were separated from one another by centrifugation and washing steps as described in the methods section. The total yield of bioexfoliated material was estimated at 35 ± 5 % when harvested after 9 days of incubation. Henceforth we will focus on the characterisation of the dispersions, which contain exclusively defluorinated GQDs; the precipitates which are made up primarily of fGO materials with varied F content and morphology are characterized separately in Supplementary Note 4.

Drop-casting of the colloid taken directly from the reaction tubes of G. sulfurreducens treated samples onto silicon wafers after biomass removal reveals a mix of large aggregates and finer particles (tens of nm) as shown in Fig. 3b and c and Figure S2 b-d. EDS mapping of these smaller particles shows that they are composed of nearly equal atomic-% C and O with an apparent total absence of fluorine (Fig. S4 and Table S4). AFM measurements of the drop-casted particles yield step height estimates on the order of 3–4 nm (Fig. 3d and e with additional data presented in Fig. S5). These results strongly suggest that incubating

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** a) Representative photographs illustrating the evolution of fluorographite after incubation with G. sulfurreducens cultures over the course of 5 days. The control tube incubated in the same medium without bacteria is shown for reference and remained unchanged over the course of the experiment. b) Biological replicate of the experiment at a 25 mL scale. The fluorographite starting material, colloid and precipitate products are illustrated for clarity. c) Close up view of the fluorographite/media interface from b). A proposed mechanism the reduction and subsequent bioexfoliation of the fluorinated material through electron transfer from the electroactive bacteria at the interface is presented.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 3. SEM images of a) fluorographite starting material b) – c) fluorographite after 7 days of incubation with G. sulfurreducens. d) AFM image of GQDs deposited on Si wafers. The height profile along with red line in d) is given in e). f) HRTEM images of purified biogenic GQDs. g) histogram of GQD particle sizes estimated by measuring 100 GQDs across different areas of the TEM grid. h)–i) HRTEM images of GQDs. The inset in i) shows a HRTEM image of a single GQD showing its crystallinity and interplanar spacing of ~0.24 nm derived from the intensity profile j) taken along the white line in i). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

fluorographite with *G. sulfurreducens* results in both its defluorination and exfoliation to form carbon nanomaterials. To further confirm that defluorination occurred, we analysed the spent incubation medium after the bacterial treatment via fluoride-sensitive potentiometry (Fig. S6). Results showed high mM concentrations of fluoride in bacteria-treated samples, with negligible concentrations measured for the pristine fluorographite.

## 3.2. Spectroscopic characterisation of biogenic nanocarbons

The colloidal materials were further separated from larger particles by centrifugation at 20000×g and spin filtration using 10 kDa MW filters as outlined in the methods. These GQDs were imaged by TEM (Fig. 3f–h, i), confirming the presence of nanoparticles <10 nm in size. The histogram of particle sizes (Fig. 3 g) gives an estimated average particle size of 3 ± 1 nm, similar to height profiles measured via AFM and consistent with GQDs. Fringes evident in high-resolution TEM (Fig. 3h and i) confirm the crystallinity of the particles; an interplanar spacing of ~0.24 nm is observed as shown in Fig. 3j, consistent with the expected value for the (1120) plane of graphite [38].

The colloidal dispersions after the removal of cells and spin filtration with 10 kDa MW cutoff filters retained the distinct pale red color characteristic of *G. sulfurreducens* cultures (Figure S7 a), which suggests the presence of cytochrome proteins. BCA assays confirmed *ca.* 50 µg/

mL protein remaining in GQD solutions after the removal of cells and biomass by centrifugation. UV-Visible spectroscopy on these crude products (Figure S7 b) showed features characteristic of c-type cytochromes. These proteins remained even using 100 kDa MW cutoff filters, consistent with outer membrane cytochromes produced by *G. sulfurreducens* which are known to self-assemble into larger structures such as protein nanowires [30,39]. We removed all proteins from these dispersions by a denaturing treatment in 2 % SDS at 80 °C followed by centrifugal filtration with 100 kDa MW filters. The purified GQD dispersions were.

then characterized by a combination of UV-Visible, FTIR, Raman and fluorescence spectroscopy. The UV-Visible absorbance spectrum of purified GQDs shown in Fig. 4 a) shows the appearance of distinct peaks at ~260 nm and ~330 nm, assigned to π→π* and n→π* transitions respectively [40,41]. The π→π* transitions are typically assigned to aromatic sp² domains while n→π* transitions are normally assigned to surface states which arise at edges functionalised with oxygen groups [42]. Abiotic controls of fluorographite incubated in the minimal media yielded only a broad absorbance close to 200 nm, similar to dispersions of the untreated starting material (Fig. S8). This suggests that the fluorinated starting material remains stable in the absence of *G. sulfurreducens*.

Raman spectroscopy of the GQDs in Fig. 4 b) shows typical D and G peak features of disordered carbon nanomaterials, while very weak
<figure><img src="image_6.png" /><figcaption>Fig. 4. a) UV–Visible spectra of a GQD solution prepared from treatment of fluorographite with *G. sulfurreducens* in PBS/Acetate media. Spectra of fluorographite incubated under identical conditions in media only are presented as a control. The inset highlights the region between 200 and 500 nm where distinct π→π* and n→π* transitions are evident in the bacteria-treated sample only. b) Raman spectrum (532 nm laser excitation) of the GQDs presented in a) and pristine fluo- rographite. c) Fluorescence excitation and emission spectra of the GQDs dispersed in ultrapure water recorded using an excitation wavelength of 250 nm. d) FTIR spectra of pristine fluorographite, precipitates after incubation with *G. sulfurreducens* and purified GQDs.</figcaption></figure>

signals were obtained for fluorographite, as expected for highly fluori-
nated carbons [11,43]. The D peak – associated to disordered features in
the carbon scaffold such as defects, edges and impurities – appears at a
typical value of ~1335 cm⁻¹ while the G peak is blue-shifted from its
typical region of ~1580 cm⁻¹ to ~1620 cm⁻¹. This shift is typical of
graphene oxides with a high degree of sp³ carbons [44] and suggests that
the GQDs are highly defective and/or oxidized [45]. This result is
consistent with the D to G peak intensity ratio, I<sub>D</sub>/I<sub>G</sub> estimated through
the deconvolution of the Raman spectra of GQDs presented in Fig. S9
and Table S5. The I<sub>D</sub>/I<sub>G</sub> is indicative of the degree of disorder in the
carbon scaffold in carbon materials. The estimated ratio of 1.7 for our
GQDs is typical of GO (see Table S6 and References [46,47]).

It is well-known that GQDs possess fluorescent properties depending
on variables such as particle size, sp² cluster size and defect states or
surface traps [42,48,49]. The fluorescent properties of the GQDs
dispersed in water are summarised in Fig. 4 c. The purified GQDs were
found to be UV-fluorescent with an emission maximum observed at 306
nm (~4.06 eV) and excitation maximum at 280 nm (4.4 eV). No sig-
nificant fluorescence was observed for control samples of fluorographite
incubated in acetate medium only (Fig. S10). This deep-UV emission
maximum is among the shortest emission wavelengths ever reported for
GQDs, with only the microwave-assisted GQDs prepared by Tang et al.
having a higher emission energy at 4.1 eV [48].

We also found that the fluorescence emission maximum of the GQDs
shifted significantly from 333 nm to 306 nm with little change in the
fluorescence intensity after the removal of cytochrome proteins by SDS
treatment (Fig. S10). An almost identical shift was found when crude
GQDs were transferred into ethanol without removing the proteins

(Fig. S11), suggesting that the fluorescence is linked to surface states
and/or solvent interactions rather than any direct contribution from the
proteins themselves.

FTIR studies were carried out on the starting fluorographite material
as well as on both the precipitates and the purified colloidal phase after
biomass removal (Fig. 4 d). The FTIR spectrum of the GQDs is consistent
with previous reports on GO materials [50], showing peaks attributed to
O–H (~3700–3000 cm⁻¹), C=O (~1700 cm⁻¹) and C=C (~1600 cm⁻¹).
Importantly, the sharp peak attributed to C–F species at around
1200 cm⁻¹ [51] is prominent in the starting material, diminished in the
precipitates and entirely absent in GQDs, suggesting fluorine elimina-
tion in the biogenic nanomaterials and the formation of new nanocarbon
materials terminated with oxygen moieties [51,52].

X-ray Photoelectron Spectroscopy measurements were used to
quantitatively follow the defluorination of the starting material and to
characterise the surface chemistry of the resulting GQDs. Survey spectra
of *G. sulfurreducens* treated fluorographite compared to abiotic controls
and the fluorographite starting material are given in Fig. 5 a). Aside from
peaks associated to the Si substrate [53], the major elements present for
the pristine material and abiotic control are C, F and O. Area ratios taken
from high-resolution scans (Fig. 5b–d-e and Fig. S12 and S13) were used
to calculate the O/C, N/C and F/C ratios summarised in Table 1. For
both pristine fluorographite and abiotic controls a single intense F 1s
peak is observed around 688 eV (Fig. 5 b). The F/C is 1.1 for the starting
material, in agreement with the SEM-EDX analysis reported in Fig. S4).
Incubating the fluorographite in 10 mM acetate medium only slightly
reduces the F/C ratio to 1.0, which is more likely attributed to adven-
titious carbon contamination than to a loss of F, since the O/C ratio of
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** a) Survey spectra of GQDs prepared by incubation of fluorographite with *G. sulfurreducens*, fluorographite incubated in media only and pristine fluorographite. Spectra are normalised by the maximum counts per second (CPS) and presented offset from one another for clarity. b) High-resolution core level spectra in the F 1s region for *G. sulfurreducens* treated fluorographite compared to control and pristine samples, showing total defluorination of the samples occurs only when co-incubated with the electroactive bacteria. c) High-resolution core-level spectra in the C 1s region of *G. sulfurreducens* treated fluorographite, compared to the media-only control and pristine sample. Peak-fitted spectra of d) pristine fluorographite and e) *G. sulfurreducens* treated fluorographite.</figcaption></figure>

<table><caption>**Table 1**

Elemental composition ratio of fluorographite and GQD samples derived from XPS analysis.</caption><thead><tr><th>Sample</th><th>C at. %</th><th>O at. %</th><th>N at. %</th><th>F at. %</th><th>O/C</th><th>N/C</th><th>F/ C</th></tr></thead><tbody><tr><td>Fluorographite</td><td>43.9</td><td>5.0</td><td>0.4</td><td>50.7</td><td>0.11</td><td>0.009</td><td>1.1</td></tr><tr><td>Abiotic Control</td><td>44.9</td><td>6.1</td><td>0.7</td><td>48.3</td><td>0.13</td><td>0.014</td><td>1.0</td></tr><tr><td><i>G. sulfurreducens</i> treated</td><td>61.9</td><td>30.9</td><td>7.2</td><td>0</td><td>0.49</td><td>0.1</td><td>0</td></tr></tbody></table>

the control remains unchanged from the value of the as-received powder of ~0.1. By contrast, high-resolution scans of the F 1s region (Fig. 5 b) for the *G. sulfurreducens* treated sample shows a total absence of F in the survey spectrum. This result was reproduced across numerous batches (Fig. S15) and confirms that the biogenic GQDs are fluorine-free. Further evidence for defluorination is found in high-resolution core-level spectra in the C 1s region (Fig. 5 c). For both untreated and abiotic control samples, the main C 1s peak is centered around 291 eV consistent with fluorinated carbons [54], while that of *G. sulfurreducens* treated fluorographite is shifted closer to 286 eV as expected for GO [55]. Peak-fitted spectra are presented in Fig. 5d and e and Fig. S12. For control and pristine samples, the predominant contributions are C–F-CF₆, C–F₂ and C–F species [56] with a lower binding energy contribution most likely due to adventitious carbon contamination. The C 1s region measured for GQDs produced by *G. sulfurreducens* treatment shows peaks centered at 284.5 eV, 286.2 eV and 287.8 eV consistent with graphitic C=C, C–O and C=O contributions respectively [55].

This result combined with the dramatic increase in the O/C from ~0.1 to ~0.5 points to a replacement of C–F surface groups primarily by oxygen moieties. Finally, the increase of the N/C from ~0.01 to 0.1 may

be due to residual proteins or organic matter arising from bacteria which remain detectable by XPS even after purification.

### 3.3. The role of riboflavin as a redox shuttle

It is well-known that C–F linkages in exfoliated fluorographene can be replaced with other moieties under relatively mild conditions [13]. In particular, the reaction of C–F with amines has been suggested as a route towards nitrogenated carbons [8,57]. Recently Siedle et al. proposed a redox mechanism for this process whereby fluorinated carbon serves as an electron acceptor and defluorination produces radical intermediates which then react with amines or other co-reactants to produce the modified carbon surface [17].

In the case of *G. sulfurreducens*, the *exo*-electrogenic redox behaviour involves membrane cytochrome proteins and external conductive protein nanowires which allow for electrons derived from intracellular respiratory processes to be transferred to external, solid electron acceptors [21,30]. It is already certain that our incubation conditions encourage the production of abundant c-type cytochromes based on UV–Visible spectra of crude colloidal products presented in Fig. S7. Considering that the media used for fluorographite incubation contains acetate as an electron donor and fluorographite as an acceptor, we hypothesise that defluorination occurs through direct electron transfer of bacteria attached at the fluorographite surface (Fig. 2 c) using these secreted cytochromes. This would explain the

visible changes in appearance of the fluorographite localised at the interface between the starting material and the *G. sulfurreducens* in solution in Fig. 2 a) and b).

However, soluble redox shuttles such as flavins can also facilitate indirect electron transfer between surface cytochromes and electron acceptors [37], without the need for bacterial adhesion. We ruled out
direct fluorographite modification by riboflavin in the absence of bacteria using media-only controls which used 10 µM riboflavin as a redox shuttle without adding *G. sulfurreducens* (see Fig. 4a and 5, and Fig. S1 and S10 in the Supporting Information). Hence, while amines can directly replace C–F in fluorinated graphenes using more aggressive treatments [8], this does not appear to occur under the mild and biocompatible conditions employed here. To investigate the contribution of riboflavin to the observed bioexfoliation, we carried out experiments using media containing only 10 mM acetate and PBS as a buffer. TEM imaging and spectroscopic analysis of riboflavin-free GQDs is reported in Fig. 6, with additional characterization reported in **Supplementary Note 3**. As Fig. 6a–d shows, we found comparable particle sizes and almost identical spectroscopic responses for GQDs prepared in media supplemented with and without the mediator. XPS analysis reported in Fig. S15 and Table S7 show comparable surface chemical composition of riboflavin-free GQDs. Hence riboflavin addition is not essential for bio-exfoliation by *G. sulfurreducens*. We observed a more rapid onset of apparent bioexfoliation when 10 µM riboflavin was present (compare the visual appearance of riboflavin-free samples in Fig. S14 with those incubated with riboflavin addition shown in Fig. 2a, both after 3 days of incubation with *G. sulfurreducens*) which indicates a

likely role in increasing the efficiency of electron transfer by redox shuttling, as has been observed for Fe(III) bioreduction by electroactive bacteria [37].

In addition to the colloidal dispersions discussed above, incubation of fluorographite with *G. sulfurreducens* also produced black precipitates at the bottom of the reaction tubes. These are characterised in Supplementary Note 4. SEM-EDX analysis of the precipitates presented in Fig. S16 shows particles ranging from tens of nm to multiple microns in size with a wide range of F/C values between 0.2 and 0.6, consistent with fGO. Structures resembling graphene nanoplatelets and other layered structures were identified in TEM images (Fig. S17), suggesting that the biogenic protocol can produce a variety of carbon nano-structures with differing surface termination and morphology without the need for ultrasonication or agitation. The presence of residual fluorine in these bioexfoliated materials ranging.

from 10 to 25 atomic % is of broad interest for the synthesis of heteroatom-modified nanocarbons, as these C–F groups can be potentially substituted with C–O, C–N or C–S linkages [6,8–10,17].

Top-down methods for the production of GO derivatives from graphite almost universally require harsh physical and chemical treatments such as ultrasonication to exfoliate graphene layers, followed by

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** a) TEM images of GQDs prepared from fluorographite in acetate media without riboflavin addition. b) HRTEM image of a single GQD taken from the bounded region shown in red in a). c) intensity profile taken along the white line in b) from which an interplanar spacing of ca. 0.22 nm was found. d) shows additional TEM images from the same batch of riboflavin-free GQDs. e) UV–Visible and f) fluorescence spectra of GQDs produced from *G. sulfurreducens* treatment of fluorographite with and without the addition of 10 µM riboflavin to the growth media. Spectra were obtained on crude GQD products without the removal of cytochrome proteins. The sample containing 10 µM riboflavin is a biological replicate of the experiments reported in Figs. 2–5. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
oxidative or thermal treatments to modify the carbon surface or to
produce smaller graphitic entities including GQDs. Fluorinated graph-
ites offer an alternative route to versatile surface modifications by
cleaving C–F, but synthetic routes to GO or GQDs reported so far require
exfoliation to fluorographene in organic solvents such as DMF, aceto-
nitrile and ethanol, followed by reflux or prolonged acid/base treat-
ments [6,8,9,58,59]. This is because fluorographite itself remains stable
indefinitely [17], a fact affirmed in our control studies, where it retains
essentially all of its F content under abiotic conditions regardless of the
media composition. To explain the rapid degradation of fluorographite
in the presence of G. *sulfurreducens*, we propose a single-step redox
mechanism catalysed by the electroactive bacteria which simulta-
neously defluorinates fluorographite while producing both GQDs and
partially-defluorinated GO materials. This mechanism, illustrated
schematically in Fig. 2 c), involves no liquid phase exfoliation methods
such as ultrasonication to delaminate fluorographene. Defluorination is
instead initiated by electron transfer to the fluorographite [13,17,60].
Previous evidence of direct reductive C–F removal was shown only at
high temperatures from 95 to 200 °C with as much as 3 % fluorine
remaining in the products [17], while another aqueous method reported
for C–F bond cleavage of fluorographite used radical chemistry to both
defluorinated the material and produce a range of nanomaterials
including nanoplatelets similar to those characterized in this work [61].

The total defluorination of fluorographite we observe at ambient
temperatures suggests that the reduction is efficiently catalysed at the
bacteria-fluorographite interface. This suggests a previously-unknown
biocatalytic role for the outer membrane proteins of *G. sulfurreducens*
in C–F cleavage. Further work is necessary to elucidate the specific roles
of proteins such as outer membrane cytochromes which are likely
involved in relaying electrons from the bacteria to the fluorographite.

Assuming fluorographite degradation proceeds through *exo*-electro-
genic respiration by bacteria in direct surface contact, the redox po-
tential of fluorographite must be sufficiently high in order to accept
electrons from membrane cytochromes, for which the formal potential
E⁰ has been estimated at ca. –200 mV [21]. There are varying reports of
the redox potential for fluorographite; Yaokawa et al. found that elec-
trochemical defluorination of CF₁₁ in fluoride-ion batteries can occur at
–0.14 V vs SHE [3] while Siedle et al. report the reduction of defective
fluorographite by decamethylosmocene at potentials as high as +0.64 V
vs SHE [17]. It is already known from studies with polarised electrodes
that *G. sulfurreducens* can develop rich biofilms at potentials as low as
–0.15 V vs SHE [62,63]. Hence it is plausible that fluorographite serves
as the terminal electron acceptor for electrons ultimately derived from
acetate through metabolism by *G. sulfurreducens*. This mechanism also
explains the presence of partially defluorinated GO materials found in
the precipitates. Reductive defluorination by the bacteria produces
partially-fluorinated fGO particles which are soluble enough to precip-
itate and subsequently undergo further reductive cutting and defluori-
nation by the bacteria to produce smaller nanostructures. In the limit
this results in ultrasmall fluorine-free GQDs.

4. Conclusions

We have presented a sustainable, biogenic route to GO from fluo-
rographite which involves the apparent biocatalysis of C–F cleavage at
low temperature and neutral pH. Direct contact between electroactive
bacteria and fluorographite is required for this bioexfoliation, suggest-
ing a catalytic role for specific outer membrane proteins in defluorina-
tion which has never previously been observed. This route eliminates
the need for organic solvents and for ultrasonication, as bioexfoliation
proceeds directly from the graphitic starting material in aqueous media.
We anticipate that this protocol will provide new, green routes to
nanocarbons with size and surface chemistry tailored to particular ap-
plications in sensing, optics and electrocatalysis.

CRediT authorship contribution statement

**Sarra Knani:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Data curation. **Riad Ahmed Boughezal:** Methodology, Investigation. **Baptiste Cibron:** Methodology, Investigation, Formal analysis, Data curation. **Frédéric Barriere:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Conceptualization. **Corinne Lagrost:** Writing – review & editing, Methodology, Investigation, Formal analysis, Data curation. **Muriel Escadeillas:** Visualization, Methodology, Investigation, Formal analysis, Data curation. **Ludivine Rault:** Writing – review & editing, Visualization, Investigation, Data curation. **Maida Costa de Oliveira:** Writing – review & editing, Validation, Methodology, Investigation. **Paula E. Colavita:** Writing – review & editing, Validation, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. **James A. Behan:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.

Acknowledgements

This work was partially supported by the “PHC ULYSSES” program (project number: 48456XA), funded by the French Ministry for Europe and Foreign Affairs and Taighde Eireann – Research Ireland. This project has received financial support from the CNRS through the MITI inter-disciplinary programs. This work was funded by the French National Research Agency (ANR-25-CE50-4529-01 HYBNA). The Raman, TEM and XPS measurements have been performed on the SIR, THEMIS and ASPHERYX platforms at UR, respectively. L. Joanny is acknowledged for SEM images performed at the CMEBA platform (ScanMAT, UAR 2025 Université de Rennes-CNRS). The authors thank C. Poriel and C. Quinton for providing access to spectroscopy instruments.

**Appendix A. Supplementary data**

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.carbon.2025.121053.

References

[1] H. An, Y. Li, P. Long, Y. Gao, C. Qin, C. Cao, Y. Feng, W. Feng, Hydrothermal preparation of fluorinated graphene hydrogel for high-performance supercapacitors, J. Power Sources 312 (2016) 146–155, https://doi.org/10.1016/j.jpowsour.2016.02.057.

[2] D. Damien, P.M. Sudeep, T.N. Narayanan, M.R. Anantharaman, P.M. Ajayan, M. M. Shaijumon, Fluorinated graphene based electrodes for high performance primary lithium batteries, RSC Adv. 3 (2013) 25702–25706, https://doi.org/10.1039/C3RA45377D.

[3] R. Yaokawa, K. Mukai, T. Nonaka, S. Kosaka, N. Ogihara, Redox reaction mechanism of graphite fluoride (CF)n in fluoride-ion batteries, Chem. Mater. 35 (2023) 10584–10593, https://doi.org/10.1021/acs.chemmater.3c02217.

[4] C. Hanmandlu, M. Sahoo, C.-C. Liu, H.-A. Chen, C.-W. Pao, Y.-C. Chang, C.-W. Chu, C.-S. Lai, Few-layer fluorine-functionalized graphene hole-selective contacts for efficient inverted perovskite solar cells, Chem. Eng. J. 430 (2022) 132831, https://doi.org/10.1016/j.cej.2021.132831.

[5] R.L. Fusaro, H.E. Sliney, Graphite fluoride (CF)x—A new solid lubricant, ASLETransact. 13 (1970) 56–65, https://doi.org/10.1080/05698197008972282.

[6] R.A. Borse, M.B. Kale, S.H. Sonawane, Y. Wang, Fluorographene and its composites: fundamentals, electrophysical properties, DFT studies, and advanced applications, Adv. Funct. Mater. 32 (2022) 2202570, https://doi.org/10.1002/adfm.202202570.

[7] X. Chen, K. Fan, Y. Liu, Y. Li, X. Liu, W. Feng, X. Wang, Recent advances in fluorinated graphene from synthesis to applications: critical review on functional chemistry and structure engineering, Adv. Mater. 34 (2022) 2101665, https://doi.org/10.1002/adma.202101665.
[8] D. Zaoralová, V. Hrubý, V. Sedajová, R. Mach, V. Kupka, J. Ugolotti, A. Bakandritsos, M. Medved', M. Otyepka, Tunable synthesis of nitrogen doped graphene from fluorographene under mild conditions, ACS Sustainable Chem. Eng. 8 (2020) 4764-4772, https://doi.org/10.1021/acssuchemeng.9b07161.

[9] M. Dubecký, E. Otyepková, P. Lazar, F. Karlický, M. Petr, K. Cěpe, P. Banáš, R. Zboril, M. Otyepka, Reactivity of fluorographene: a facile way toward graphene derivatives, J. Phys. Chem. Lett. 6 (2015) 1430–1434, https://doi.org/10.1021/acs.jpclett.5b00565.

[10] D.D. Chronopoulos, A. Bakandritsos, M. Pykal, R. Zboril, M. Otyepka, Chemistry, properties, and applications of fluorographene, Appl. Mater. Today 9 (2017) 60–70, https://doi.org/10.1016/j.apmt.2017.05.004.

[11] R.R. Nair, W. Ren, R. Jalil, I. Riaz, V.G. Kravets, L. Britlitt, P. Blake, F. Schedin, A. S. Mayorov, S. Yuan, M.I. Katsnelson, H.-M. Cheng, W. Strupinski, L.G. Bulusheva, A.V. Okotrub, I.V. Grigorieva, A.N. Grigorenko, K.S. Novoselov, A.K. Geim, Fluorographene: a two-dimensional counterpart of teflon, Small 6 (2010) 2877–2884, https://doi.org/10.1002/smll.201001555.

[12] R. Zboril, F. Karlický, A.B. Bourlinos, T.A. Steriotis, A.K. Stubos, V. Georgakilas, K. Safárová, D. Jančík, C. Trapalis, M. Otyepka, Graphene fluoride: a stable stoichiometric graphene derivative and its chemical conversion to graphene, Small 6 (2010) 2885–2891, https://doi.org/10.1002/smll.201001401.

[13] K.E.Jr. Whitener, R. Stine, J.T. Robinson, P.E. Sheehan, Graphene as electrophile: reactions of graphene fluoride, J. Phys. Chem. C 119 (2015) 10507–10512, https://doi.org/10.1021/acs.jpcc.5b02730.

[14] J. Sturala, J. Luxa, M. Pumera, Z. Sofer, Chemistry of graphene derivatives: synthesis, applications, and perspectives, Chem. Eur. J. 24 (2018) 5992–6006, https://doi.org/10.1002/chem.201704192.

[15] V. Hrubý, D. Zaoralová, M. Medved', A. Bakandritsos, R. Zboril, M. Otyepka, Emerging graphene derivatives as active 2D coordination platforms for single-atom catalysts, Nanoscale 14 (2022) 13490–13499, https://doi.org/10.1039/D2NR03453K.

[16] W. Feng, P. Long, Y. Feng, Y. Li, Two-dimensional fluorinated graphene: synthesis, structures, properties and applications, Adv. Sci. 3 (2016) 1500413, https://doi.org/10.1002/advs.201500413.

[17] A.R. Siedle, Y. Losovyy, J.A. Karty, D. Chen, K. Chatterjee, V. Carta, B.D. Stein, U. Werner-Zwanziger, C-F bond activation in the solid state: functionalization of carbon through reactions of graphite fluoride with amines, J. Phys. Chem. C 125 (2021) 10326–10333, https://doi.org/10.1021/acs.jpcc.1c00734.

[18] X. Ye, L. Ma, Z. Yang, J. Wang, H. Wang, S. Yang, Covalent functionalization of fluorinated graphene and subsequent application as water-based lubricant additive, ACS Appl. Mater. Interfaces 8 (2016) 7483–7488, https://doi.org/10.1021/acsmami.5b10579.

[19] L. Yang, Y. Li, L. Wang, Y. Pei, Z. Wang, Y. Zhang, H. Lin, X. Li, Exfoliated fluorographene quantum dots as outstanding passivants for improved flexible perovskite solar cells, ACS Appl. Mater. Interfaces 12 (2020) 22992–23001, https://doi.org/10.1021/acsmami.0c04975.

[20] T. Philippon, F.-Z. Ait-Itto, A. Monfort, F. Barrière, J.A. Behan, Fe(III) oxide microparticles modulate extracellular electron transfer in anodic biofilms dominated by bacteria of the pelobacter genus, Bioelectrochemistry 151 (2023) 108394, https://doi.org/10.1016/j.bioelechem.2023.108394.

[21] T. Ueki, Cytochromes in extracellular electron transfer in geobacter, Appl. Environ. Microbiol. 87 (2021), https://doi.org/10.1128/AEM.03109-20, 03109-20.

[22] G. Ceriotti, A. Bosco-Santos, S.M. Borisov, J.S. Berg, Microbial iron reduction under oxic conditions: implications for subsurface biogeochemistry. https://doi.org/10.48550/arxiv.2404.07137, 2024.

[23] T. Shi, C. Peng, L. Lu, Z. Yang, Y. Wu, Z. Wang, A. Kappler, Response of Fe(III)-reducing kinetics, microbial community structure and Fe(III)-related functional genes to Fe(III)-organic matter complexes and ferrihydrite in lake sediment, Biogeochemistry 167 (2024) 1553–1565, https://doi.org/10.1007/s10533-024-01186-4.

[24] J.V. Boas, V.B. Oliveira, M. Simões, A.M.F.R. Pinto, Review on microbial fuel cells applications, developments and costs, J. Environ. Manag. 307 (2022) 114525, https://doi.org/10.1016/j.jenvman.2022.114525.

[25] J.M. Byrne, N. Klueglein, C. Pearce, K.M. Rosso, E. Appel, A. Kappler, Redox cycling of Fe(II) and Fe(III) in magnetite by Fe-metabolizing bacteria, Science 347 (2015) 1473–1476, https://doi.org/10.1126/science.aaa4834.

[26] C. Bryce, N. Blackwell, C. Schmidt, J. Otte, Y. Huang, S. Kleindienst, E. Tomaszewski, M. Schad, V. Warter, C. Peng, J.M. Byrne, A. Kappler, Microbial anaerobic Fe(II) oxidation – ecology, mechanisms and environmental implications, Environ. Microbiol. 20 (2018) 3462–3483, https://doi.org/10.1111/1462-2920.14328.

[27] L. Liu, C. Zhu, M. Fan, C. Chen, Y. Huang, Q. Hao, J. Yang, H. Wang, D. Sun, Oxidation and degradation of graphitic materials by naphthalene-degrading bacteria, Nanoscale 7 (2015) 13619–13628, https://doi.org/10.1039/C5NR02502H.

[28] C. Zhu, L. Liu, M. Fan, L. Liu, B. Dai, J. Yang, D. Sun, Microbial oxidation of graphite by Acidithiobacillus ferrooxidans CFMI-1, RSC Adv. 4 (2014) 55044–55047, https://doi.org/10.1039/C4RA09827G.

[29] J. Wang, Q. Ma, Z. Zhang, S. Li, C.S. Diko, C. Dai, H. Zhang, Y. Qu, Bacteria mediated Fenton-like reaction drives the biotransformation of carbon nanomaterials, Sci. Total Environ. 746 (2020) 141020, https://doi.org/10.1016/j.scitotenv.2020.141020.

[30] Y. Gu, M.J. Guberman-Pfeffer, V. Srikanth, C. Shen, F. Giska, K. Gupta, Y. Londer, F.A. Samatey, V.S. Batista, N.S. Malvankar, Structure of geobacter cytochrome OmCZ identifies mechanism of nanowire assembly and conductivity, Nat. Microbiol. 8 (2023) 284–298, https://doi.org/10.1038/s41564-022-01315-5.

[31] F. Wang, C.H. Chan, V. Suciu, K. Mustafa, M. Ammend, D. Si, A.I. Hochbaum, E. H. Egelman, D.R. Bond, Structure of geobacter OmCZ filaments suggests extracellular cytochrome polymers evolved independently multiple times, eLife 11 (2022) e81551, https://doi.org/10.7554/eLife.81551.

[32] E.C. Salas, Z. Sun, A. Lüttge, J.M. Tour, Reduction of graphene oxide via bacterial respiration, ACS Nano 4 (2010) 4852–4856, https://doi.org/10.1021/nn101081t.

[33] G. Wang, F. Qian, C.W. Saltikov, Y. Jiao, Y. Li, Microbial reduction of graphene oxide by shewanella, Nano Res. 4 (2011) 563–570, https://doi.org/10.1007/s12274-011-0112-2.

[34] C.E.A. Engel, D. Vorländer, R. Biedendieck, R. Krull, K. Dohnt, Quantification of microaerobic growth of Geobacter sulfurreducens, PLoS One 15 (2020) e0215341, https://doi.org/10.1371/journal.pone.0215341.

[35] C. Sun, Y. Feng, Y. Li, C. Qin, Q. Zhang, W. Feng, Solvothermally exfoliated fluorographene for high-performance lithium primary batteries, Nanoscale 6 (2014) 2634–2641, https://doi.org/10.1039/C3NR04609E.

[36] M.C. Vu, N.A. Thi Thieu, J.-H. Lim, W.-K. Choi, J. Chan Won, MdA. Islam, S.-R. Kim, Ultrathin thermally conductive yet electrically insulating exfoliated graphene fluoride film for high performance heat dissipation, Carbon 157 (2020) 741–749, https://doi.org/10.1016/j.carbon.2019.10.079.

[37] J.M. Byrne, H. Muhamadali, V.S. Coker, J. Cooper, J.R. Lloyd, Scale-up of the production of highly reactive biogenic magnetite nanoparticles using Geobacter sulfurreducens, J. R. Soc. Interface 12 (2015) 20150240, https://doi.org/10.1098/rsif.2015.0240.

[38] H. Koolivand, S. Mazinani, F. Sharif, Surface activity of N-graphene quantum dot in presence of CTAB surfactant, Surf. Interfaces 37 (2023) 102714, https://doi.org/10.1016/j.surfin.2023.102714.

[39] K. Inoue, X. Qian, L. Morgado, B.-C. Kim, T. Mester, M. Izallalen, C.A. Salgueiro, D. R. Lovley, Purification and characterization of OmCZ, an outer-surface, octaheme c-Type cytochrome essential for optimal current production by Geobacter sulfurreducens, Appl. Environ. Microbiol. 76 (2010) 3999–4007, https://doi.org/10.1128/AEM.00027-10.

[40] C. Zhu, S. Yang, G. Wang, R. Mo, P. He, J. Sun, Z. Di, Z. Kang, N. Yuan, J. Ding, G. Ding, X. Xie, A new mild, clean and highly efficient method for the preparation of graphene quantum dots without by-products, J. Mater. Chem. B 3 (2015) 6871–6876, https://doi.org/10.1039/C5TB01093D.

[41] Y. Li, Y. Hu, Y. Zhao, G. Shi, L. Deng, Y. Hou, L. Qu, An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics, Adv. Mater. 23 (2011) 776–780, https://doi.org/10.1002/adma.201003819.

[42] S. Lai, Y. Jin, L. Shi, R. Zhou, Y. Zhou, D. An, Mechanisms behind excitation- and concentration-dependent multicolor photoluminescence in graphene quantum dots, Nanoscale 12 (2020) 591–601, https://doi.org/10.1039/C9NR08461E.

[43] U. Rajeena, M. Akbar, P. Raveendran, R.M. Ramakrishnan, Fluorographite to hydroxy graphene to graphene: a simple wet chemical approach for good quality graphene, New J. Chem. 42 (2018) 9658–9665, https://doi.org/10.1039/C8NJ01392F.

[44] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud'homme, I.A. Aksay, R. Car, Raman spectra of graphite oxide and functionalized graphene sheets, Nano Lett. 8 (2008) 36–41, https://doi.org/10.1021/nl071822y.

[45] D. López-Díaz, M. López Holgado, J.L. García-Fierro, M.M. Velázquez, Evolution of the raman spectrum with the chemical composition of graphene oxide, J. Phys. Chem. C 121 (2017) 20489–20497, https://doi.org/10.1021/acs.jpcc.7b06236.

[46] K.K.H. De Silva, P. Viswanath, V.K. Rao, S. Suzuki, M. Yoshimura, New insight into the characterization of graphene oxide and reduced graphene oxide monolayer flakes on Si-Based substrates by optical microscopy and Raman spectroscopy, J. Phys. Chem. C 125 (2021) 7791–7798, https://doi.org/10.1021/acs.jpcc.1c01152.

[47] E. Dervishi, Z. Ji, H. Htoon, M. Sykora, S.K. Doorn, Raman spectroscopy of bottom-up synthesized graphene quantum dots: size and structure dependence, Nanoscale 11 (2019) 16571–16581, https://doi.org/10.1039/C9NR05345J.

[48] L. Tang, R. Ji, X. Cao, J. Lin, H. Jiang, X. Li, K.S. Teng, C.M. Luk, S. Zeng, J. Hao, S. P. Lau, Deep ultraviolet photoluminescence of water-soluble self-passivated graphene quantum dots, ACS Nano 6 (2012) 5102–5110, https://doi.org/10.1021/nn007606E.

[49] H. Qin, T. Gong, Y. Jin, Y. Cho, C. Shin, C. Lee, T. Kim, Near-UV-emitting graphene quantum dots from graphene hydrogels, Carbon 94 (2015) 181–188, https://doi.org/10.1016/j.carbon.2015.06.033.

[50] V. Brusko, A. Khannanov, A. Rakhmatullin, A.M. Dimiev, Unraveling the infrared spectrum of graphene oxide, Carbon 229 (2024) 119507, https://doi.org/10.1016/j.carbon.2024.119507.

[51] A. Wang, S. Bok, C.J. Mathai, R. Thiruvengadathan, C.M. Darr, H. Chen, M. R. Zachariah, K. Gangopadhyay, J.A. McFarland, M.R. Maschmann, S. Gangopadhyay, Synthesis, characterization and nanoenergetic utilizations of fluorine, oxygen co-functionalized graphene by one-step XeF2 exposure, Combust. Flame 215 (2020) 324–332, https://doi.org/10.1016/j.combustflame.2020.02.005.

[52] F. Zhou, Q. Dong, J.-T. Chen, B. Sengupta, J. Jiang, W.L. Xu, H. Li, S. Li, M. Yu, Printed graphene oxide-based membranes for gas separation and carbon capture, Chem. Eng. J. 430 (2022) 132942, https://doi.org/10.1016/j.cej.2021.132942.

[53] D.S. Jensen, S.S. Kanyal, N. Madaan, M.A. Vail, A.E. Dadson, M.H. Engelhard, M. R. Linford, Silicon (100)/SiO2 (100), XPS, Surf. Sci. Spectra 20 (2013) 36–42, https://doi.org/10.1116/11.20121101.

[54] R. Stine, W.-K. Lee, K.E. Whitener, J.T. Robinson, P.E. Sheehan, Chemical stability of graphene fluoride produced by exposure to XeF2, Nano Lett. 13 (2013) 4311–4316, https://doi.org/10.1021/nl4021039.
[55] X. Chen, X. Wang, D. Fang, A review on C1s XPS-spectra for some kinds of carbon materials, fullerenes, nanotubes and, Carbon Nanostruct. 28 (2020) 1048–1058, https://doi.org/10.1080/1536383X.2020.1794851.

[56] A.P. Dementjev, A.V. Eletskii, K.I. Maslakov, E.G. Rakov, V.F. Sukhoverhov, A. V. Naumkin, Fluorination of carbon nanostructures and their comparative investigation by XPS and XAES spectroscopy, fullerenes, Nanotube. Carbon Nanostruct. 14 (2006) 287–296, https://doi.org/10.1080/15363830600663990.

[57] H. Li, G. Hou, X. Yuan, Z. Liu, W. Luo, Y. Song, S. Bi, KOH modification of fluorinated graphite and its reaction mechanism, Front. Mater. 9 (2022), https://doi.org/10.3389/fmats.2022.999753.

[58] P. Gong, K. Hou, X. Ye, L. Ma, J. Wang, S. Yang, Synthesis of highly luminescent fluorinated graphene quantum dots with tunable fluorine coverage and size, Mater. Lett. 143 (2015) 112–115, https://doi.org/10.1016/j.matlet.2014.12.058.

[59] P. Gong, Z. Yang, W. Hong, Z. Wang, K. Hou, J. Wang, S. Yang, To lose is to gain: effective synthesis of water-soluble graphene fluoroxide quantum dots by sacrificing certain fluorine atoms from exfoliated fluorinated graphene, Carbon 83 (2015) 152–161, https://doi.org/10.1016/j.carbon.2014.11.027.

[60] W. Lai, Y. Yuan, X. Wang, Y. Liu, Y. Li, X. Liu, Radical mechanism of a nucleophilic reaction depending on a two-dimensional structure, Phys. Chem. Chem. Phys. 20 (2017) 489–497, https://doi.org/10.1039/C7CP06708A.

[61] A. Sahu, J.R. Alston, C. Carlin, M. Craps, K. Davis, H.B. Harrison, T. Kongruengkit, A. Manikonda, S. Elmore, R. Rollins, B. Sadiku, S. Schmal, J. Shajahan, J.C. Poler, Fluorographite nanoplatelets with covalent grafting of anion-exchange resins for water purification, ACS Appl. Nano Mater. 5 (2022) 5709–5721, https://doi.org/10.1021/acsanm.2c00709.

[62] C.I. Torres, R. Krajmalnik-Brown, P. Parameswaran, A.K. Marcus, G. Wanger, Y. A. Gorby, B.E. Rittmann, Selecting anode-respiring bacteria based on anode potential: phylogenetic, electrochemical, and microscopic characterization, Environ. Sci. Technol. 43 (2009) 9519–9524, https://doi.org/10.1021/es902165y.

[63] A.S. Commault, G. Lear, M.A. Packer, R.J. Weld, Influence of anode potentials on selection of geobacter strains in microbial electrolysis cells, Bioresour. Technol. 139 (2013) 226–234, 10/f45c3k.