Citrus limetta Organic Waste Recycled Carbon Nanolights

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Citrus limetta Organic Waste Recycled Carbon Nanolights: Photo-electro catalytic, Sensing and Biomedical Applications Anupma Thakur, Pooja Devi, Shefali Saini, Rishabh Jain, Ravindra Kumar Sinha, and Praveen Kumar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04025 • Publication Date (Web): 20 Nov 2018 Downloaded from http://pubs.acs.org on November 21, 2018

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ACS Sustainable Chemistry & Engineering

Citrus limetta Organic Waste Recycled Carbon Nanolights: Photo-electro catalytic, Sensing and Biomedical Applications Anupma Thakur1, 2, #, Pooja Devi2, #,*, Shefali Saini2, Rishabh Jain1 , Ravindra Kumar Sinha2, and Praveen Kumar3,* 1Academy 2Central 3Department

of Scientific and Innovative Research (AcSIR), Ghaziabad, India-201002

Scientific Instruments Organisation, Sector 30-C, Chandigarh, India-160030 of Materials Science, Indian Association for the Cultivation of Science, Kolkata, India-700032

Corresponding authors: [email protected] (Pooja Devi) and [email protected] (Praveen Kumar)

ABSTRACT The present work reports green route-waste recycled carbon nanolights i.e. carbon dots (GCDs) synthesized via a facile one-step pyrolysis method from Citrus limetta waste pulp. The size of these obtained pristine GCDs is ~4-7 nm (HR-TEM), with high optical and structural quality as revealed by FT-IR and Raman spectroscopic analysis. They exhibit the highest quantum yield of 63.3 % over other similar green synthesized GCDs, favourable for many applications. Further, we demonstrate the multifunctional aspects of these synthesized GCDs for photoelectrochemical water splitting, photocatalytic methylene blue degradation, Fe(III) ions sensing, bactericidal activity (against E. coli and S. aureus), and bioimaging with excellent performance. The visible light active characteristic of GCDs is observed to achieve an efficient current density of ~6 mA/cm2 towards water splitting. This study demonstrates the waste to wealth potential of recycled waste derived GCDs in wide range of application domains. Keywords: GCDs, Waste Recycle, Green Synthesis, Water Splitting, Biomedical, Dye Degradation.

# Equally contributed

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INTRODUCTION Carbonaceous nanostructures including carbon nanotubes (CNTs), graphene, carbon nanofibres (CNFs), fullerene, nano-diamonds (NDs), carbon dots (CDs), graphene quantum dots (GQDs), etc., have engrossed a significant attention in the research domain over the past few decades. This is because of their excellent physiochemical and relatively better opto/electronic properties to their other counterpart such as metallic nanostructures, semiconductor quantum dots (SQDs), organic fluorophores, dyes, conjugated polymers, etc.1,2. CDs amongst this class, being the smallest and brightest member of the family, has found tremendous applications in the domain of optical chemosensors, biosensors, bio-imaging (bacterial and human/cell lines), fluorescent ink development, opto/electronics devices, and drug delivery. Further, they are also most investigated photosensitizer after plasmonic nanostructures in the field of photocatalytic dye degradation, photocatalytic/electrochemical water splitting, solar devices, visible light bactericidal activity, etc.3,4. This is because of their photostability, chemically inertness, cost-effectiveness, non-toxicity, eco-friendliness, biocompatibility, colloidal stability, and tunable photoluminescence properties5. Therefore, over the years several processes have been established for their synthesis from a variety of sources and using different “top-down” and “bottom-up” approaches. These mainly include arc discharge6, acidic oxidation7, laser ablation8, microwave pyrolysis9, electrochemical oxidation10, thermal combustion11, and hydrothermal oxidation12 of carbon precursors. Most of these approaches involve complicated multistep fabrication, tedious setup, expensive precursors & processes, post-synthesis separation, acidic synthesis environment, laborious procedures, etc., which has put down the economic viability and environmental acceptability of the reported synthesis routes13. Further in-situ doping with other heteroatoms such as sulphur, nitrogen, transition metals, etc., or post-synthesis surface modification is done for their surface passivation to obtain high quantum yield (QY). Green synthesis on the other hands 2 ACS Paragon Plus Environment

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eliminates the need of using expensive and toxic chemicals to achieve the same goal, since it relies

upon

using

natural

sources

consisting

of

carbohydrate,

proteins,

lipids,

macro/micronutrients, etc. Therefore, these green approaches have gained the remarkable interest of the research community for synthesizing high quality & tunable CDs, which embrace the tendency of being the subsequent generation's green nanomaterials to substitute conventional one for similar applications. In consideration of the above facts, green carbon dots (GCDs) synthesis is reported from carbonization of fruit peels (e.g., watermelon, pomelo) 14-16, whey (diary waste) 17, dried lemon peel 18, coffee grounds 19, forestry and agricultural biomass 20, sago waste 21, frying oil 22, waste chimney oil 23, cigarette filters 24, pork 25, cellulose waste paper 26, urine 27, Citrus grandis 28, etc. The literature survey in the recent year shows an increasing trend towards recycling waste foods/materials into GCDs through carbonization (ESI, Figure S1). This is due to their abundant and inexpensive availability and associated benefits as complementary means for waste management. However, the challenge remains in GCDs synthesis is to obtain high QY, without using any external dopant, which is highly dependent upon the initial precursor and synthesis procedure. It is therefore desirable to establish new processes and explore new precursors to obtain fluorescent GCDs with high QY to make them commercially viable for wide applications encroached by CDs. Citrus limetta deliberated to be a cultivar of Citrus limon, is a species of citrus, also commonly known as sweet lemon, is rich in citric acid, vitamins, minerals and sugars (including sucrose, glucose, and fructose). Henceforth, the pulp waste can exclusively act as a carbon source as well as a passivating agent for GCDs. Herein, the current effort presented in this manuscript adds to the autonomy of previously reported research work in the green-synthesis routes for GCDs, by reporting a facile approach for GCDs synthesis from Citrus limetta pulp waste through one step pyrolysis under optimized conditions. A detailed analysis of optical and 3 ACS Paragon Plus Environment


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physicochemical properties of resulting GCDs has been presented in detail. The pulp driven GCDs exhibit excellent and stable fluorescent properties with a very high QY of 63.3 % over other similar reports, which manifest their promising use in several applications. Since GCDs are popularly known to act as photogenerated electrons collectors and transporters, therefore, we present the first report on GCD to use them as an efficient photosensitizer in photoelectrochemical (PEC) water splitting. TiO2 is a widely used photoelectrode in water splitting, however, due to its narrow bandgap and excitons recombination, several visible light sensitizers such as metal nanoparticles, SQDs, dyes, biomolecules, etc., are reported to broaden the light absorption to increase the overall PEC efficiency. Few reports on chemically synthesized CDs based composites such as H:ZnO-nanorod/CDs29, TiO2 nanotube arrays/CDs30,

NiFe-layered

double-hydroxide

nanoplates/CDs31,

CDs/Co3O4-Fe2O332,

NiOOH/FeOOH/CDs/BiVO433, Ti/CDs@α-Fe2O334, etc. are available in PEC application. However, to the best of our knowledge GCD/TiO2 is not yet been investigated, which can be a potential cost-effective substitute for existing similar composite for clean fuel generation. Further, the proposed composite is also studied for photocatalytic dye degradation using methylene blue (MB) as a model dye. Since GCDs contains several surface functionalities such as hydroxyl, carboxyl, carbonyl, etc., we, therefore, used these waste driven GCDs for metal ions sensing, which are the major inorganic pollutants of concern worldwide. Since the bactericidal infection is amongst the largest cause of death worldwide, we also report our GCDs for visible light active bactericidal activity against both Gram-positive and negative bacterium, owing to visible light induced exciton in GCDs,3,35-37. While CKK8 assay and Glioblastoma cell lines (U373 and U87MG) were studied for their cytotoxicity behaviour and bio-imaging potential, respectively.

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EXPERIMENTAL SECTION Materials GCDs were synthesized using Citrus limetta (known as mossami fruit in India) waste pulp as a sole carbon precursor. The photo-catalytic/electrochemical activities of GCDs were investigated by loading them onto TiO2 nanofibers (TNFs), which were prepared with electrospinning technique using polyvinylpyrrolidone (PVP, M.W. 1,300,000), acetic acid, ethanol, as reactants, and titanium tetraisopropoxide (Ti[OCH(CH3)2]4, Loba Chemie) as precursors. The QY of obtained GCDs was calculated against quinine sulphate (QS, CDH, India) as reference dye. MB dye (Loba Chemie) was studied as a model dye for GCDs mediated degradation under light. While PEC studies of GCDs/TNF photoelectrodes were carried out in sodium hydroxide (1M, pH 9) electrolyte. Ethanol, acetic acid, and acetone procured from Merck were used as a solvent. Standard stock solutions (1000 mg/L) of various ions including Cu(II), Ni(II), Pb(II), Cd(II), Cr(III), Zn(II), Mg(II), Li(II), As(III), Co(II), Fe(III) were purchased from Merck and Inorganic Venture. Deionized water collected from Millipore corporation grade water system (Milli-Q, 18.2 MΩ) was used in the preparation of all reagents and standard solutions. Synthesis of GCDs and TNFs GCDs were synthesized by an optimized pyrolysis route by means of a hot air oven for the procedure and pulp waste as a precursor. The parametric optimization in terms of synthesis time and the temperature was carried out earlier by our group 38. Therefore, optimal conditions (Temp=190˚C, Time=20 minutes) in the oven were used for the CDs synthesis from the waste pulp for the first time. A pale yellow thick residual collected at the end of synthesis was dissolved in deionized water and stored at 4°C until used. During the pyrolysis step, the carbonaceous natural components of Citrus limetta passes through dehydration,



polymerization, and carbonization processes sequentially, followed by in-situ surface passivation under optimized synthesis condition. The presented method of synthesis is facile, single step, fast and requires inexpensive carbon source as well as instrumentation. TNFs were synthesized using a precursor solution consisting of ethanol (7 mL), acetic acid (7 mL) and titanium tetraisopropoxide (4 mL) and an ethanolic solution of PVP (5 wt. %). Thus, prepared solution was transferred to a syringe (15.6 mm diameter), further employed in the electrospinning apparatus. An optimized potential of 12 kV was used to obtain TNF, which were later collected onto the substrate (ITO: 1 × 2.1 cm2, sheet resistance: < 15 Ω) as well as in crucible. The as-spun nanofibers were calcined in a muffle furnace for half an hour at 673K for removal of PVP polymer and achieve crystallization of TNFs. GCDs were loaded onto thus obtained TNFs by overnight incubation for 20h in dark condition, which were later rinsed with ultrapure deionized water, and air dried. For PEC water splitting studies, the GCD/TNF were fabricated by electrospinning TNFs onto ITO glasses and later incubated overnight in as prepared solution of GCDs solution under dark conditions. These GCDs on TNFs form discontinuous porous islands, which reduces the reﬂection of incident light and assist desorption of generated gases which can enhance the PEC performance due to more incident light penetration. Characterization The optical characteristics of synthesized GCDs, TNFs, and GCD/TNFs were probed with UVVis (Hitachi, U-3900-H) and fluorescence (PL, Carry-Varian) spectrophotometer. The bright blue fluorescence of GCDs was observed under UV cabinet (POPULAR INDIA) and well photographed. Dialysis membrane (Spectrum Labs) with molecular weight cut-off 0.3kDa was used for the purification of pristine GCDs. The surface functionalities and carbonaceous structure of GCDs were tapped by FT-IR (Varian 670 IR) and Raman (inVia instrument,


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Renishaw, λex= 785 nm) measurements, while HR-TEM (H-7500, Hitachi Ltd., Japan) for morphological and structural characterizations. PEC study with GCDs/TNFs photoelectrodes The PEC water splitting capability of the GCDs/TNFs was investigated under 1-sun irradiation using standard three-electrode configuration and computer controlled electrochemical workstation (AUTOLAB, Metrohm). GCDs/TNF coated onto ITO was used as a photoanode (working electrode), Pt wire and Ag/AgCl were used as counter and reference electrodes, respectively. Sodium hydroxide (1 M, pH 9) was used as a supporting electrolyte, while solar simulator (LOT Quantum design) with an illumination intensity of 100 mW/cm2 was used as light source. Current and Impedance spectra of the fabricated photoelectrodes were recorded under dark and illumination condition at 0 V vs Ag/AgCl with an AC perturbation signal of 10 mV. Mott-Schottky (MS) measurements of the fabricated electrodes were recorded in 1 M NaOH in frequency-potential scan mode at 100 kHz. To probe the response of GCDs as a visible sensitizer, the experiments were performed using AUTOLAB potentiostat, equipped with light emitting diodes (LEDs) (blue, green, red and yellow) photo-kit. Photocatalytic activity of GCDs/TNFs The photocatalytic activities of the GCDs/TNFs were investigated under visible light irradiation against MB dye. For photocatalytic dye degradation study, the synthesized catalysts (20 mg) was added into MB dye solution (10 mg/l, 20 mL), and stirred for 30 minutes in dark condition to achieve adsorption-desorption equilibrium. The solution was then subjected to UV-Visible light irradiation (250 W lamp) and samples were collected at an interval of five minutes until complete degradation of dye, monitored through UV-Vis spectrophotometer. The collected sample aliquots were first centrifuged to remove GCDs/TNF and then analyzed for MB characteristics. Likewise, GCDs and TNFs alone were used for control experiments.



Langmuir-Hinshelwood model

39

was used to calculate dye degradation rate, while the

concentration of dye was determined with respect to the model developed for MB dye absorption (at 664 nm) vs. concentration calibration equation. Metal Ion sensing with GCDs The obtained pristine GCDs were investigated against a range of inorganic ions such as Cu(II), Ni(II), Pb(II), Cd(II), Cr(III), Zn(II), Mg(II), Li(II), As(III), Co(II), etc., wherein their cocktails (1 ppm) were prepared from 1000 ppm standard solutions. The metal ions of 1 ppm were mixed with GCD solution at a ratio of 1:1 and incubated. The preliminary screening exhibited the high selectivity of GCDs towards Fe(III) ions, observed as a reduction in fluorescence intensity w.r.t decrease in Fe(III) concentration. The emission data was collected at λexc=360 nm with a slit width of 10 nm for an optimized sensor to analyte ratio being 1:1. The F/F0, representative of a change in fluorescence intensity corresponds to the ratio of GCDs fluorescence intensity in the presence (F) and absence (F0) of Fe(III) ions varied concentrations. The selectivity performance of the pristine GCDs was also tested against a library of ions commonly present in water. Cytotoxicity and bioimaging studies of GCDs CKK8 assay was performed to assess the cytotoxicity of GCDs. Glioblastoma cell lines U373 and U87MG were seeded into 96 well plate at a density of 1 x 104 cells per well. After 4 hrs of seeding, the GCDs at different concentrations (as 0, 0.01, 0.05, 0.1, 0.2, 0.5, 1.0 mg/mL) were added to cells in triplicate. Cells were grown with GCDs for 24 and 48 hrs respectively. The cell culture media containing GCDs was removed and the cells were washed three times with PBS. 20L CKK8 was added in 200 µL of media and incubated for 1hr in the CO2 incubator. Then the media containing CKK8 was removed and 150 L of DMSO was added. Optical density (OD) was read at 450 nm using a plate reader (Tecan life science). Cell viability was


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measured as (OD sample/OD control)*100. The cellular toxicity experiments were conducted in three replicates. Fluorescence imaging of cell lines treated with GCDs was also performed. U373 and U87MG glioblastoma cell lines were seeded onto chamber slide at a concentration of 2 x 104 cells per well overnight in the CO2 incubator. GCDs (20 g / mL) were added to the cells along with control. Cells were incubated with GCDs for 2 hrs followed by removal of media containing GCDs and washing thrice with PBS. Cells were fixed using 4% paraformaldehyde for 15 minutes in media. Cells were washed with PBS and visualized under a confocal microscope using different fluorescent filters (Nikon A1 laser scan confocal microscope). Light-activated bactericidal activity of GCDs The bactericidal activity of the as-synthesized GCDs induced by light was investigated using the disk diffusion method. E. coli and S. aureus cells were used in the experiments to evaluate the antibacterial activities of photoexcited GCDs. For disk diffusion test, 200 µL of different concentrations of GCDs i.e. 1 mM, 0.1 M, 0.3 M, and 0.6 M (No. 1 to 4) were loaded into wells perforated in agar medium. The plates were incubated at an optimal temperature of 35˚C for a period of 24 hrs under dark and visible light (36 W CFL light bulb at a height of 30 cm) conditions. The plates were read for zones of inhibition in reference to GCDs concentration and light/dark conditions. Also, the OD at 600 nm of E. coli and S. aureus cultures incubated under light/dark conditions with different concentration of GCDs was recorded. RESULTS AND DISCUSSION Characterization of GCDs Figure 1(a) presents the optical (absorption and emission) characteristics recorded for the pristine GCDs. It is found that the absorption spectrum exhibit a broad absorption peak at ~350 nm, which is generally assigned to n→π* and π →π* molecular transitions observed in C-C 9 ACS Paragon Plus Environment


molecules in CDs architecture7. While the photoexcited emission (λex = 360 nm) studies depict the appearance of broad emission characteristics (370 - 550 nm) with maximum intensity centered at ~430 nm. The origin of luminescence in GCDs is assigned to the presence of sp2 clusters in the core40. Moreover, the effect of excitation wavelength (λex = 300-400 nm) on the emission spectrum of GCDs manifest their excitation independent emission behavior (Figure 1(b)), which is further explained in terms of uniform core centers within GCDs architecture and is in-line with the previous reports41. The QY of obtained pristine GCDs was calculated by relating the integrated fluorescence intensity (excited at 350 nm) and the absorbance value of GCDs at 350 nm with reference to quinine sulphate dye (0.01 M H2SO4 (QY=54%)) and was found to be ~63.3 % (ESI, Figure S2), which is highest amongst earlier reports on GCDs synthesized from natural precursors (ESI, Table S1). The high QY of GCDs makes them a promising candidate in sensing, imaging, and other related applications. Further, in contrast, we have also characterized the Citrus limetta waste pulp to probe its absorption, emission, and surface functionalities. The results (ESI, Figure S3) demonstrate that the extract does not reveal any absorption in mid-UV and visible range, and furthermore has no traces of n→π* and π →π* molecular transitions. The emission spectrum does not show any significant PL spectra, reveals their non-luminescent nature. Additionally, the FT-IR of Citrus limetta extract reveals the presence of various functional groups such as for C=C, C-H stretch of aromatic hydrocarbons and –OH surface groups. FT-IR and Raman spectroscopic techniques were further used to probe their surface functionalities. Figure 1 (c) reveals the presence of various functional groups specific characteristic vibration modes such as for C=C stretch of aromatic hydrocarbons, C=O stretch of oxygenic and carboxylated surface groups, which marks them rich in hydrophilicity in nature42. Likewise, D and G bands at ~1372 cm-1 and ~1638 cm-1, respectively in the Raman spectrum of GCDs (Figure 1(d)) confirms their carbonaceous


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architecture. The presence of relatively high intensity of D band over G band in spectrum also indicates the increased degree of localized sp3 defects within sp2 clusters43. A plausible mechanism for the formation of GCDs from Citrus limetta waste pulp is illustrated in scheme 1. During the pyrolysis step, the carbonaceous natural components of Citrus limetta passes through dehydration, followed by the polymerization, and carbonization processes sequentially leading to nucleation of the carbonized structure into GCD cores by in-situ surface passivation under optimized synthesis condition. The final outcome is GCDs with bright blue luminescence and abundant hydroxyl and carboxyl groups on their surface as indicated by FTIR and Raman measurements. These GCDs readily got dispersed in water and can easily be purified before use. Figure 2 portrays the morphological and structural characteristics of synthesized GCDs captured with HR-TEM, where the image analysis (Figure 2 (a) and (b)) reveals that the obtained GCDs are of spherical morphology with an average diameter of ~ 4-7 nm. The dynamic light scattering (DLS) results are also in support of the HR-TEM (ESI, Figure S4). The lattice fringe analysis shows parallel graphitic lines of d spacing 0.31 nm .31 nm), while SAED pattern of GCDs (inset, Figure 2(c)) depicts their amorphous nature, which also supports Raman analysis discussed above. The structure of the GCDs and GCDs/TNF electrode is further probed by XRD measurements and results are presented in Fig 2(d). The measurements confirm the amorphous nature of GCDs, whereas for GCDs/TNF composite it show associated planes namely (101), (004) and (211) for TiO2 for the GCDs/TNF electrode44,45. The peak related to Si(111) is related to the substrate. The obtained GCDs showed bright blue luminescence under UV-illuminator, while the physical appearances of the synthesized GCDs suspensions were yellowish-brown w.r.t. synthesis condition. Further, the UV-Vis spectra of TNF and GCD/TNF composite (ESI, Figure S5) shows that TNFs did not show any absorption



in the visible region, while GCD/ TNF composite shows an extended absorption tail starring from UV to visible region. PEC water splitting The synthesized GCDs presents as an abundant source as a photosensitizer for PEC watersplitting for hydrogen production. Before utilizing them in PEC, the fabricated GCD/TNF and TNF electrodes were electrochemically characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to understand charge transfer characteristic at the electrode surface. The CV studies were conducted using Ferri-Ferro (5mM in 0.25 M KCl) redox probe due to its well-reported redox characteristic as a function of electrode/electrolyte interface. The defined redox peaks w.r.t. Fe2+/Fe3+ species were observed from CV spectra (ESI, Figure S6). Further, with an increase in scan rate from 10 to 100 mV/sec, an increased anodic (ipa) and cathodic (ipc) peak current was observed for GCD/TNF electrodes over TNF electrodes alone. Moreover, EIS characteristics of fabricated electrodes (ESI, Figure S6) recorded in 105 to 0.1 Hz frequency range are also in line with CV measurements. The Randel’s equivalent circuit fit (Inset, Figure S6) for obtained Nyquist plots of TNF and GCD/TNF was analyzed to calculate charge transfer (Rct) resistance. The lower value of Rct for GCD/TNF (16.3 Ω) over TNF (32.4 Ω) could be understood by GCDs charge transfer characteristic through unsaturated carbon units within GCDs structure46. The negative slope in M-S spectrum (ESI, Figure S7) of GCD/TNF photoanode depicts their n-type semiconductor characteristics47. The PEC water splitting studies of photo-electrodes in 1M NaOH solution showed a high current density of ~ 5.6 mA/cm2 for GCD/TNF over TNF ( ~ 1 mA/cm2) working-electrode under 1 sun illumination, as shown in Figure 3 (a). The small current plateau at  +1.0 eV (vs RHE) can be due to the mass transfer process. Aimed at the distinct roles of water as both solvent and reactant, the influence of mass transfer process is frequently analysed earlier48-50. The value is larger at higher pH value as reported in literature43, indicating the OH12 ACS Paragon Plus Environment

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concentration is significant in the mass transfer process. Moreover, the over-potential of 130 mV at 2 mA/cm2 for GCDs/TNF electrode, we have selected +1.1 V (vs RHE) as a bias potential for PEC to achieve excellent water splitting activity under 1M NaOH (pH 9) environment. The impedance spectra (Figure 3 (b) & (c)) recorded under similar condition revealed the decrease in Rct value for GCD/TNF electrode to ~111 Ω from ~140 Ω for TNF, confirming higher diffusion of channelized electrons under light illumination for GCD/TNF photoelectrodes. The observed high current density and reduced Rct values of GCD/TNF could be assigned to the presence of GCDs onto UV active n-type TNFs. The GCDs can act as an organic photosensitizer, in withdrawing and channelizing the additional photo-excited electrons to the conduction band of the TNF, subsequently, it prolongs the visible light reactivity inherited from GCDs of the modeled UV-Vis active semiconductor i.e. TiO2 and thereby, enhancing overall water splitting performance as presented in scheme 1.

The

photoresponse study of GCD/TNF photoelectrode was further carried out in chopped-LSV experimentation at 1.1 V (vs RHE) under light and dark condition and results are portrayed in Figure 3 (d). The photoelectrode exhibit negligible photocurrent density under dark condition, which increases instantly and reaches to a steady state under light condition. After the illumination source is turned off, the photocurrent reaches back to the background level, and this process can be repetitive many times. However, the initial decrease (spike) in the current density can be due to the carrier accumulation in TNF before they start channelling through GCDs, which further gets stabilized. During the OFF condition, the slow approach towards zero can be related to the possible defects at the TNF and GCDs interface51,52. The energy band gap of GCDs, TNF and GCD/TNF were measured to be 2.6 eV, 3.2 eV, and 2.8 eV, respectively (ESI, Figure S9). Further, on illumination with monochromatic LEDs for GCDs/TNF photoanode, we observed the highest response for blue light (ESI, Figure S8), which can be 13 ACS Paragon Plus Environment


due to the extended range of light response. This extended response further can be attributed to the existence of the electronic coupling between π states of GCDs and the conduction band of TNF32,53 and which in-turn enhances the water reduction for H2 generation. The generated holes in the valence band of TNF transfer to the valence band of GCDs, and these accumulated holes are engaged in water oxidation. Photocatalytic activity The photocatalytic activity of GCD/TNF composite was investigated against MB dye under UV-Visible light irradiation (250 W lamp). The characteristic peak of MB (663 nm) was followed up to assess the dye degradation performance of the composite 54. Figure 4 (a) & (b) presents absorption (spectrophotometric and colorimetric) characteristic of the dye in the presence of GCD/TNF catalyst at different time intervals. Upon an increase in irradiation time, the color as well as absorption peak intensity of the dye decrease. The change in absorption intensity i.e. (Ao-AT/Ao), w.r.t. composite and controls (TNF and GCD alone) reveal that the photocatalytic performance of composite is higher than controls (Figure 4 (c)). The % degradation shown in Figure 4(d) divulges that GCD and TNF alone could only degrade 75% and 82% of the dye even after 30 minutes, while complete degradation is observed for GCD/TNF as a catalyst for same time duration. The substantial increase of the photocatalytic activity of GCD/TNF towards MB dye degradation under visible light irradiation is attributed to their wider light response as compared to GCDs or TNF alone. Under sunlight irradiation, GCDs being a visible light absorber allows the enhanced absorption of solar components of the visible region while the shorter wavelengths of UV regime excite TNFs to produce an electronhole pair that further increases photocatalytic spot at GCD/TNF interface. The main contribution of GCD loaded onto TNF is as an electron collector and transporter. The generated electron-hole pairs under light irradiation are trapped by the surface of the catalyst to produce OH- radicals. The hydroxyl radical oxidizes dye into other non-harmful by-products. 14 ACS Paragon Plus Environment

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Additionally, the dissolved O2 molecules encountered with the excited electrons also produces superoxide radical anions, O2−, to further speed up dyes degradation55. In order to assess the self-degradation of MB dye in the presence of visible light, the experiments were executed without using the GCD/TNF catalyst and the self-degradation of MB was found fairly constant (ESI, Figure S10). Also, TNF and GCD alone couldn’t degrade the dye completely (ESI, Figure S11). The reusability of the catalysts was also assessed (ESI, Figure S12). The present catalyst has shown improved dye degradation characteristic over previously reported GCD/TNF

56,

wherein GCD of QY ~14 % were synthesized by a complex process from lemon peel. The high performance with Citrus limetta pulp waste driven GCD could be due to their high QY (~63.3 %). Ion Sensing GCDs are significantly investigated in metal ions sensing owing to their high aspect ratio and availability of a large number of functionalities on their surface. The metal ion binding event is reported to change their properties, in particular, optical properties. In lieu of these facts, we have investigated these as prepared and water-soluble GCDs of high QY for a library of ions comprising of Cu(II), Ni(II), Pb(II), Cd(II), Cr(III), Zn(II), Mg(II), Li(II), As(III) and Co(II) under similar condition, and the results are presented in Figure 5 (a). It is observed that Fe(III) ions have the maximum static fluorescence quenching effect on GCDs fluorescence amongst other metal ions. This further indicates that these GCDS are highly sensitive and selective for the detection of Fe(III) ions as compared to other metal ions. The slight quenching observed for Cu(II) ions can be understood in terms of nonspecific electrostatic interactions between GCDs carboxylic groups and Cu(II) ions. With the above information, the detailed sensing response of GCDs towards Fe(III) ions was studied and discussed further. Figure 5 (b) shows the effect of Fe(III) ions concentration on GCDs fluorescence intensity. The change in fluorescence intensity i.e. F/Fo exhibit a fashioned linear decrease w.r.t. increased the 15 ACS Paragon Plus Environment


Page 16 of 31

concentration of Fe(III) ions in the 50-100 ppb range. Further, GCDs were also tested against a higher concentration range of Fe(III) ions (Figure 5(c)) and the observed sensing behavior is theoretically modeled with the following equation (1):

{

―5.8𝑒 ―3𝑥 + 1.257 𝑓𝑜𝑟 50 < 𝑥 ≤ 100 ―3.08𝑒 ―4𝑥 + 0.711 𝑓𝑜𝑟 300 < 𝑥 ≤ 700 Proposed Mo𝑑𝑒𝑙; 𝑦 = ―2.51𝑒 ―5𝑥 + 0.355 𝑓𝑜𝑟 1000 < 𝑥 ≤ 10000 ……………..(1) where y is F/Fo and x is Fe(III) concentration. The above results support the practical utility of the synthesized probe for Fe(III) ions sensing in a wide concentration range, while meeting the WHO guidelines of the lower limit. The quenching tendency of the GCDs probe towards Fe(III) was estimated at a S/N ratio and a fitting parameter of 3 and 0.978, respectively, by means of Stern-Volmer plot. At optimized conditions, the Stern-Volmer equation states the quenching in fluorescence instigated by the addition of analyte i.e. Fe(III), as per the following equation (2): 𝐼𝑂 𝐼

= 𝐾𝑆𝑉(𝑄) + 𝐶 …………….. (2)

where I0 and I correspond to the fluorescence intensity in the absence and presence of quencher (analyte i.e. Fe (III)), respectively, KSV is the constant for Stern−Volmer, representing the affinity between GCDs as fluorescent probe and Fe(III) as quencher, Q represents the analyte concentration, and C is a constant (generally 1). For a linear regression equation, the Ksv, was found to be 5.11 x 108 M in the linear concentration range of 50-100 ppb as presented in Figure 5 (d). The analytical performance metrics such as theoretical detection limit and limit of quantification was calculated to be 19.8 ppb and to be 60 ppb, respectively, which is far below the WHO guidelines of 300 ppb and satisfies the drinking water quality criterion. Prominently, these as-prepared GCDs contain characteristic surface carboxylated groups that have distinct binding affinity towards to Fe(III) ions causing recombination of charge carrier and hence static


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quenching observed in fluorescence of GCDs. Table 1 compares the analytical performance of the presented probe

57-54, 23

against other reported green synthesized CDs for Fe(III) ions

sensing. The excellent analytical characteristics of this probe could be understood in terms of its high quantum yield, to enable wide detection range of Fe(III) ions down to 19.8 ppb being limit of detection that fulfills the WHO guidelines. Cytotoxicity study and bio-imaging applications of GCDs The CDs are known to exhibit high resistance to photobleaching and inherent tunable fluorescence properties, so the bioimaging potential of the synthesized GCDs was also explored. Fluorescence imaging of U373 and U87MG cells incubated with 20 l/mL GCDs under the confocal microscope showed different emissions under different filters (Figure 6 (a)). Green, blue and red emission was observed under FITC (495 - 519 nm), TRIT-C (475 - 490 nm), DAPI (340 - 380 nm), respectively. While there was basal minimum fluorescence in control cells, Cells incubated with GCDs exhibited bright fluorescence inside cells. The results are consistent with the earlier studies showing cell imaging property of GCDs where they penetrate inside the cells and show different emission behaviour under different excitation wavelengths. The cytotoxicity study has also shown no toxic effect of GCDs on cells even after 24 and 48 hrs of incubation, and even at the highest concentration of 1 mg/mL (Figure 6 (b). There was no difference in viability observed between the control and cells seeded with different concentration of GCDs (0.01 – 1.0 mg/mL). This result further supports their biocompatible (non-toxic) nature for their potential use as an excellent bioimaging and environmentally acceptable material for wide applications. Antimicrobial activity of GCDs Due to increased bacterial infection, the quest for an alternative bactericidal agent is amongst the major topic of research. CDs as a bactericidal has engrossed the attention of the researchers



over the last few years due to their visible light active optical properties. CDs are reported to show bactericidal activity under visible light due to photoinduced exciton generation, which can rupture the bacterial cells. CD derived from edible mushrooms (Pleurotus spp., QY-25%) is shown for their bactericidal activity against E. coli.

58

In another similar work, cow milk

derived CDs showed the excellent biocidal effect against both Gram-positive (S. aureus) and Gram-negative (E.coli) bacteria when conjugated with silver nanoparticles i.e. (CDs/Ag)59. Fascinated by these reports, we also investigated the bactericidal potential of CLWP derived GCDs against E. coli and S. aureus, and the results are shown in Figure 7 (a) and (b). The appearance of the zone of inhibition in bacterial culture plates containing GCDs and placed under visible light confirmed their light activated bactericidal activity. While under dark condition, no effect was observed. The growth curve of both E.coli and Staphylococcus aureus was well studied before investigating the effect of GCDs concentration on bacterial growth monitored by measuring their OD at 600 nm. GCDs of various concentrations (0.6 M, 0.3 M, 0.1 M, and 1 mM) have shown a significant effect on OD under light conditions as observed in Figure 7 (c). As shown, GCDs -expressed antimicrobial effect and completely inhibited the growth of E.coli and Staphylococcus aureus at a MIC value of 0.1 M and 0.3 M, respectively. However, the bactericidal effect of GCDs is observed until 1mM, here lower concentration also contributed significantly to the bactericidal effect. The result also supports the observations in disk diffusion assay discussed earlier. The observed light-induced bactericidal activity of GCDs is explained in terms of their capacity to absorb light, inducing the generation of electron-hole pairs. The plausible mechanism corresponds to the reaction between electronhole pairs and O2 or OH-, that give rise to active oxygen species, which are able to react with DNA, cell membranes, and cellular proteins, leading to bacterial cell causing their death60.


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CONCLUSIONS In conclusion, the present work reports highly luminescent (QY 63.3 %) carbon dots by recycling of Citrus limetta pulp waste in a facile one-step pyrolysis method and simple setup. The obtained GCDs from cost-effective and widely available precursors are loaded onto electrospun TiO2 nanofibers to obtain GCD/TNF composite, which is investigated for the first time for photoelectrochemical water splitting. The PEC performance of GCDs based photo electrodes exhibited their promising potential in solar energy harvesting for clean fuel generation. The composite is further demonstrated to degrade methylene blue model dye with fast degradation rate of ~ 30 minutes and increase in degradation efficiency over TNF alone. Likewise, obtained label-free GCDs showed their detection capability towards Fe(III) ions in a wide linear range and with a detection limit of 19.8 ppb. The bactericidal and bio-imaging potential of GCDs is also investigated revealing their immense biomedical and environmental prospects due to their antibacterial properties and non-toxicity, respectively. AUTHOR INFORMATION Corresponding Authors * [email protected] (Pooja D.) and [email protected] (Praveen Kumar)

AUTHOR CONTRIBUTIONS The work was conceptualized and supervised by PD and PK. The experimental studies for the synthesis of GCDs and GCDs/TNF along with their applications for PEC water splitting, photocatalytic methylene blue degradation, and Fe(III) ions sensing was done by AT, whereas SS has performed the bactericidal activity, and bioimaging expirments. RJ and RKS have helped in data interpretation and analysis. PD, AT, and PK have formulated and drafted the manuscript and all authors have given final approval for this version.



NOTE The authors declare no competing financial interests. ACKNOWLEDGMENTS PD acknowledge the financial support received from the Department of Science and Technology (DST) through SEED (GAP 375) grant and AT acknowledges INSPIRE Fellowship (DST). Authors acknowledge, S. Bagchi, for providing electro-spunned TNF. ASSOCIATED CONTENT Supporting Information (QY measurement and DLS analysis of GCDs; UV-Vis absorption and XRD spectrum of GCD/TNFs; Electrochemical characterization of GCD/TNFs photo electrod (CV, EIS and Mott-Shottky); Photocurrent response of GCD/TNF electrodes in monochromatic LEDs; Dye degradation performance of GCD/TNFs composite. REFERENCES (1) (2) (3) (4) (5) (6)

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Table 1: Analytical performance comparison of GCD towards Fe (III) ions. S/n

Precursor

Quantum Yield (%)

1

Rose-heart radish

13.6

LR:10 ppb to 2.23 ppm; LOD:0.1 ppb

49

2

Papaya

18.9

LOD: 0.016 ppm

50

3

Honey

19.8

LR: 2.7 ppb to 5.58 ppm; LOD: 0.9 ppb

51

4

Egg white

31

LR: 2.79 to 13.96ppm

52

5

Lycii Fructus

17.2

LR: 10 to 1.67 ppm; LOD: 1.7 ppb

53

7.6

LOD: 1.05 ppb

54

7.5

LR: 2.5 ppb to 33.5 ppm; LOD:0.51 ppb

20

6 7

Corn Stalk Waste Chimney Oil

Analytical Performance

Reference

LOD: 19.3 ppb; 8

Citrus limetta

63.3

LR:50-100 ppb Wide range of detection:50 ppb to 10 ppm


This work


Figure 1: Optical characterizations of GCDs, (a) Absorption & emission, (b) effect of excitation wavelength on GCDs emission characteristics, (c) FT-IR, and (d) Raman spectrum

Figure 2: (a) HR-TEM, (b) particle-size distribution, (c) lattice fringe analysis, and XRD spectrum of (d) pristine GCDs and (e) GCDs/TNF. 26 ACS Paragon Plus Environment

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Figure 3:, (a) Linear sweep voltammogram (LSV) (potential vs RHE) of TNF and GCD/TNF photo electrodes under dark and 1-sun illumination, (b) & (c) impedance spectra of TNF (b) and GCD/TNF (c) photoelectrodes under illumination and (d) Chopped-LSV of photoelectrodes under dark and light condition at 1.1 V vs RHE.

Figure 4: (a) Absorption spectra of dye (10 ppm) at varied time interval on incubation with GCD/TNF catalyst (1 mg/mL) for degradation study,(b) representative images of MB dyes collected at different time intervals,(c) degradation efficiency (where A is absorbance of Dye under light (AT) and dark condition (AO) of MB dye with different catalyst, and (d) representative degradation efficiency of different catalyst w.r.t. irradiation time. 27 ACS Paragon Plus Environment


Figure 5: (a) Sensing behaviour of GCDs towards various ions, (b) Effect of Fe(III) ion concentrations on the fluorescence intensity of GCDs, (c) Regression fit for relation between fluorescence intensity (F/Fo) and Fe(III) concentration, and (d) Stern-Volmer’s plot with varied concentration of Fe(III).

Figure 6: (a) The fluorescence image of glioblastoma cell lines (U373 and U87MG) incubated with GCD (20 g / mL, 2 hrs) and (b) In vitro cell viability of cells treated with various concentrations of CDs (24 hrs) as estimated by CKK8 assay.


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Figure 7: Illustrative images of bacterial culture plates of (a) E. Coli (b) S. aureus (containing GCDs)under dark and light condition, and (c) OD600 measurements of bacterial cultures incubated with GCDs (0.6 M, 0.3 M, 0.1 M and 1 mM) under light condition for MIC measurement.

Scheme 1: Mechanism of PEC water splitting with GCD/TNF electrodes



Scheme 2: Mechanism of PEC water splitting with GCD/TNF electrodes


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Table of Contents “Waste to Wealth” : Citrus limetta Waste Derived Carbon Dots in Multimodal Applications.


Citrus limetta Organic Waste Recycled Carbon Nanolights

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