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Article Cite This: ACS Omega 2018, 3, 9096−9104
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Nanostructured Graphene Oxide Dots: Synthesis, Characterization, Photoinduced Electron Transfer Studies, and Detection of Explosives/Biomolecules Venkatesan Srinivasan,† Mariadoss Asha Jhonsi,*,† Murugavel Kathiresan,‡ and Arunkumar Kathiravan*,§ †
Department of Chemistry, B. S. Abdur Rahman Crescent Institute of Science and Technology, Chennai 600 048, Tamil Nadu, India Electroorganic Division, CSIR-Central Electrochemical Research Institute, Karaikudi 630 003, Tamil Nadu, India § Department of Chemistry, Vel Tech Rangarajan Dr Sagunthala R & D Institute of Science and Technology, Avadi, Chennai 600 062, Tamil Nadu, India ACS Omega 2018.3:9096-9104. Downloaded from pubs.acs.org by 185.89.101.36 on 08/14/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: Herein, we report the preparation of graphene oxide dots (GO dots) by fine-tuning the carbonization degree of citric acid. The structure of GO dots was characterized by absorption spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, as well as high-resolution scanning electron microscopy and transmission electron microscopy analyses. The typical particle size of the GO dots was 42 nm. The fluorescent characteristics of the GO dots were analyzed by fluorescence spectroscopy. Once excited at 360 nm, the GO dots were fluorescent in the range of 450−550 nm, which was dependent on the excitation wavelength. Further, GO dots were effectively utilized for multifarious applications such as photoinduced electron transfer and detection of explosives and biomolecules. The emission property of GO dots was competently quenched by viologens, picric acid (PA), and bilirubin (BR). The mechanism of quenching by viologens and explosives/ biomolecules was found to be due to photoinduced electron transfer and the internal filter effect, respectively. Intriguingly, the detection minimum of PA is in the nanomolar level. Toward commercialization, the economic test strips have also been introduced for the identification of PA. Furthermore, the GO dots have been applied as an efficient luminescent bioprobe for a selective and perceptive finding of BR.
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INTRODUCTION Starting from tooth paste to satellite and water purification to medicine, the use of nanomaterials has dramatically increased in recent years. Hence, it is necessary to have a basic knowledge of nanotechnology at all levels.1 By a text book explanation, nanomaterials are in the size range of 1−100 nm at least in one dimension and small in size, which results in distinctive chemical and physical properties.2 Recently, two emerging carbon-based nanomaterials,3−6 such as graphene oxide (GO)7 and graphene quantum dots (GQDs),8 were introduced into the carbon-based nanomaterial family, which are fascinating to many researchers. GO is an atomically thin sheet of graphite covalently bonded with oxygen-containing functional groups, either on the basal plane or at the edges.9 GQDs are a few graphene layers connected together in a quasispherical shape in the size of 1−10 nm,10 which exhibit exclusive characteristics owing to their quantum confinement effects. It is previously reported that GQDs show an excellent fluorescent nature when compared to GO; however, later it was described that GQDs and GO have the similar luminescence properties,8 which are highly utilized in several emerging fields, such as solar devices,11 biocellular imaging,12 and drug release.13 GQDs and GO were generally prepared via © 2018 American Chemical Society
both top-down (carving graphite crystallites via high-resolution electron beam lithography,14 cutting of GO hydrothermally,15,16 repeated oxidation,17 electrochemical methods,18 chemical oxidation treatment of carbon precursors,19,20 and cage opening of the C60 on a ruthenium exterior21) and bottom-up methods. However, the former one has some limitations, such as special types of equipment being required, less product yield, complex synthetic conditions, and poor control of the size. At the same time, the bottom-up methods such as the hydrothermal method, direct pyrolysis, and so forth are simple and economic for the preparation of graphene-based nanomaterials from special organic precursors.22 The bottomup method has some specific benefits such as defined control over the morphology and the particle size allocation of the products derived. In recent times, fluorescent carbon nanomaterials prepared from citric acid (CA) via the thermal pyrolysis route with a high fluorescence quantum yield were reported.9,23 Behrmann and Hofmann used CA precursors with ammonia and Received: May 30, 2018 Accepted: July 25, 2018 Published: August 14, 2018 9096
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metal toxicity of chalcogenide quantum dots hampers the usage of metal nanoclusters and quantum dots, which provide a pathway to use luminescent carbon dots for the efficient quantification of BR. In this study, we present GO dots, a mixture of GQDs and GO, synthesized by fine-tuning the carbonization extent of a common organic precursor, such as CA, via direct pyrolysis using a simple bottom-up approach. The prepared GO dots were successfully used for PET studies with structurally similar viologen derivatives (Scheme S1) such as ethyl viologen (EV), dimeric ethyl viologen (EDV) and trimeric ethyl viologen (ETV).51 Moreover, the bright fluorescent nature of the GO dots has been used as a chemical sensor toward sensitive and selective finding of an explosive such as PA at a nanomolar scale via luminescence quenching analysis. Further, a biologically important molecule such as BR has also been detected using GO dots as a fluorescent sensor.
described the formation of a blue emissive molecule, citrazinic acid, for the first time.24 Later, Sell and Easterfield prepared the same by reacting CA and urea at 130 °C.25 In recent times, Yang’s group explored the generation of a fluorescent citrazinic acid-derived compound by the hydrothermal method from CA with the combination of ethylenediamine.26 Three different amine precursors were used to produce carbon dots via hydrothermal treatment by Rogach et al.27 In recent times, blue luminescent GQDs and GO were prepared from CA simply by tuning the carbonization degree under similar reaction conditions.9 It is understood that the multiple oxygenbased functional groups present in GQDs and GO impart a negative charge on the surface, which interacts noncovalently with certain materials. For example, the Purkayastha research group offered the preparation of CA-based CDs and the use of surface-encapsulated derivatives of α-cyclodextrin-functionalized carbon dots with methyl viologen (MV2+) to study the prominent photoinduced electron transfer (PET) by aggregated nanostructures.28 In another upcoming field of research, the perceptive and careful detection of explosives that have been paid more attention is contemporary because of the inferences in motherland safety, worldwide demining, and ecological protection.29−31 2,4,6-Trinitrophenol (TNP), a chemical compound commercially known as picric acid (PA), is a dynamic explosive compared to the well-known trinitrotoluene.32 It has been broadly utilized in the manufacture of lethal weapons, preparation of rocket fuels, coloring agents, crackers, and light matches.33 In addition, a large amount of used PA is continuously left over in the debris remains as a major environmental pollutant.34 Hence, in the current scenario, there is a high demand for the invention of a novel and facile route for the selective finding of PA. The fluorescence quenching method has diverse benefits, such as it can be easily used, it is sensitive and economic, and it has quick response time,35−38 which makes it a facile method for the detection of explosives. A range of fluorescent materials such as organic dyes and polymers,39 metal−organic frameworks,43 quantum dots,40 assembled monolayers,41 and fluorescent carbon dots were used as high-performance fluorescence sensors.42 Among them, there are limited reports available on graphene-based fluorescent materials for quantitative identification of nitro explosives; however, unfortunately, they are a step behind in selectivity.43−46 The fluorescence quenching method has also been used for the detection of various biomolecules. Bilirubin (BR, Scheme S3) is one such important biomolecule whose level in the blood plasma acts as a marker for the detection of certain diseases; elevated levels of BR in the human body create a situation called hyperbilirubinemia. The limited range of BR in blood serum is 19.80 mg mL−1 in a newborn and 1.19 × 10−2 mg mL−1 in adults.47 Excess BR in the human body may be due to the heme catabolism and is excreted via bile, a symptom of jaundice. The high level of BR accumulation can be identified by a yellow tint of the skin and sclera in connection with jaundice.48 Moreover, an excess level of BR may cause neurotoxicity in newborns, which leads to abnormalities in neurodevelopment. Different procedures are in practice to detect BR, such as diazo, peroxidase, and separation-based methods. Along with these methods, fluorometric analysis is an easy, quick procedure and simple to detect BR using nanoclusters, quantum dots, and fluorescent carbon dots.49,50 However, the requirement of expensive precursors and heavy
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RESULTS AND DISCUSSION Material Characterization. It was reported earlier that different structured carbon nanomaterials can be acquired just by fine-tuning the degree of carbonization duration. For instance, GQDs and GO sheets were derived from CA by altering the degree of carbonization time by 1/2 and 2 h, respectively.9 In this work, an effort has been made in view of checking the characteristics of carbon nanomaterials in the case of the carbonization time fixed to 1 h. Therefore, we have done the pyrolysis of CA for a period of 1 h. Further, it is necessary to confirm the structure of the carbon nanomaterial synthesized under this experimental condition, whether the prepared material is GQDs or GO sheets. The one processed in this study is readily soluble in H2O, brownish in color, and exhibited high storage stability of more than 3 months without agglomeration. The Fourier transform infrared (FT-IR) analysis is done to find out the surface functional groups and reactive sites of the carbon nanomaterial (Figure S1a), which exhibited the O−H and N−H stretching vibration of the carboxylic acid and amine groups at 3327 cm−1 as a wide peak. The carbonyl stretching frequency was found at 1729 cm−1, confirming the existence of the carboxylic group. At 1544 and 1382 cm−1, the peaks were assigned to the carbonyl stretching and the asymmetric stretching of the C−O−C band, respectively. In the spectrum of CA, the C−H stretching is observed in the range of 1350 cm−1, and the absence of this peak in the prepared nanomaterial and the decrease in the percentage of transmittance are indicative of the fact that CA has been carbonized. The appearance of negatively charged hydroxyl and carboxy functional groups on the carbon surface was confirmed by zeta potential analysis, which explored the value of −15.3 mV (Figure S2). The combined results from both the FT-IR and zeta potential data validate the presence of various reactive sites on the surface of the prepared nanomaterial, and possibly they impart hydrophilicity. The functional groups were further analyzed by the NMR analysis. From the 1H NMR data (Figure S3), signals were observed in three regions, such as sp3 C−H protons at 1−2 ppm; ether, carbonyl, and hydroxyl protons at 3−5 ppm; and aromatic sp2 and aldehydic protons at 7−8.5 ppm. The existence of aliphatic, aromatic, and carbonyl carbons was further confirmed by the 13C NMR analysis, which shows a signal from 30 to 45 ppm corresponding to aliphatic (sp3) carbons, 100−180 ppm corresponding to aromatic or sp2 carbon, and 212 ppm corresponding to carbonyl carbon, respectively. The 9097
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discussed NMR results clearly demonstrate the ranges of functional groups present on the prepared carbon nanomaterial surface. Figures S1b and S4 show the high-resolution scanning electron microscopy (HR-SEM) images of the prepared carbon nanomaterial, which clearly indicate the existence of a layered structure and quasispherical particles. The energy-dispersive Xray spectroscopy (EDAX) analysis (Figure S5) indicates the presence of 41.31% carbon and 28.83% oxygen. This clearly proves that the prepared nanomaterial is oxygenous carbon and that the existence of other elements such as Na and Cl in the range of 25.88 and 3.98%, respectively, is due to the addition of NaOH and HCl during the separation and purification processes (neutralization). The high-resolution transmission electron microscopy (HR-TEM) image reveals that the nanomaterial consists of sheets and most of the particles are quasispherical in shape (Figure 1a) with the typical particle size in the range of 42 nm derived from the histogram plot (Figure 1b). The selected area electron diffraction (SAED) pattern (Figure 1c) in which the spherical and bright spots are exposed indicated the existence of both amorphous and crystalline particles. Raman spectroscopy measurement is an essential tool especially for the characterization of nanocarbon, which is used to prove the surface structure, crystallinity, disorder/defects, stability, dispersibility, and so forth. The Raman spectrum (Figure S6) revealed a broad D-band and a G-band at 1371 and 1610 cm−1, respectively. It is known that the sp3-hybridized orbital of a carbon atom is represented by the D band and the sp2 carbon atom by the G band. Further, the D band looks wider than the G band. Moreover, the intensity of the G band is higher than that of the D band. The calculated ratio of relative intensity of ID/IG is around 0.30, which clearly illustrates that a good quality carbon material is prepared and that the atoms are arranged in a regular fashion. This result is in good agreement with the reported Raman spectrum for GQDs; at the same time, it is also noticed that there is a shift of 30 cm−1 in the position of the G band that is endorsed to the oxygenation of some percentage of carbon, which is in line with GO.52 Therefore, from the above characterizations, we consider that the prepared carbon material exhibits the mixed behavior of both GQDs and GO. Hence, we assigned the prepared nanomaterial as GO dots. The combined behaviors of GO dots were further confirmed by optical characterizations. Optical Characterization. The UV−vis absorption spectrum of GO dots (Figure 2a) shows a broad shoulder in the range of 250−500 nm arising from the π−π* (raised from the aromatic sp2 graphitic carbon) and n−π* transitions (raised from the carbonyl group), which are similar to GO. It exhibits a strong fluorescence emission at 464 nm at the excitation of 360 nm. Further, the emission spectra were verified at the different excitation range from 280 to 480 nm in an interval of 20 nm (Figure 2b), which explored the excitation-dependent wavelength emission. The different energy levels created by the various emissive sites and fewer defects on the surface of GO dots may be the reason behind the excitation-dependent emission nature.53 Figure 2c shows that the fluorescence decay of the GO dots is fitted with the triexponential function [F(t) = A1 exp(−t/τ1) + A2 exp(−t/τ2) + A3 exp(−t/τ3)], and the lifetimes of τ1 = 0.59 ns (19%), τ2 = 2.57 ns (66%), and τ3 = 7.44 ns (15%) were obtained. The average lifetime is calculated as τ = 2.89 ns. The several lifetimes imply the existence of a mixture of emissive states on
Figure 1. (a) HR-TEM image of GO dots, (b) histogram plot drawn from HR-TEM image details, and (c) SAED patterns of GO dots.
the surface of the GO dots. The above discussions are not enough to confirm the structure of GO dots as both GQDs and GO may exist together and both of them show a similar behavior. In order to get a clear picture of the structure of GO dots, we have adopted the spectral analysis in the presence of a strong reducing agent such as NaBH4. As described in the literature,9 the optical characteristics of GQDs are not affected by NaBH4, but the properties such as absorption wavelength, absorbance, and emission intensity of GO could be altered by NaBH4. Therefore, we have analyzed the consequence of NaBH4 addition on the absorption and emission spectrum of GO dots, which is shown in Figure S7a,b. The results specified that the addition of a reducing agent leads to a reduction in the optical density and an increase in the emission intensity. These 9098
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electron donor in the excited state, and three structurally related viologen derivatives such as EV, EDV, and ETV with varying positive charges were used as electron acceptors.56,57 The UV−vis absorption analysis was conducted to study the presence of a ground-state interaction between the GO dots and the viologen derivatives. It is observed that the absorption band of the GO dots has not altered in the presence of ETV (Figure S8). The presence of other two viologens also resulted in a similar spectral behavior, which clearly points out that there is no significant ground-state interaction between the GO dots and viologen derivatives. The excited-state interaction between the GO dots and viologen derivatives has also been examined via fluorescence analysis. The GO dots in solution exhibited a strong emission at 464 nm during the excitation at 360 nm. Here, the acceptor molecules (viologen) do not absorb at 360 nm, so one can selectively excite the GO dots. The emission spectrum of the GO dots was recorded in the absence and presence of ETV and is displayed in Figure 3. The emission intensity of the GO
Figure 3. Fluorescence spectrum of GO dots (λexi: 360 nm and λemi: 464 nm) with various concentrations of ETV (0−5 mM) in a water medium.
dots was regularly diminished with an increase in the concentration of ETV, revealing the quenching. A similar type of quenching behavior was observed for other viologen derivatives such as EV and EDV, and the spectra are displayed in Figure S9a,b. The extent of luminescent quenching is described by the Stern−Volmer (S−V) eq 1
Figure 2. (a) Absorption, excitation, and emission spectra, (b) excitation-dependent emission spectrum, and (c) fluorescence decay of GO dots (λexi: 370 nm).
I0/I = 1 + KSV[Q ]
changes are due to the addition of NaBH4, which has the capability to remove some of the oxygen atoms from the sp3 C−O environment (including C−OH, C−O−C, and COOH) to form an sp2 domain. The formed sp2 domain consists of new repeating units, similar size allocation, and surface states, which leads to an increase in the emission intensity. These results demonstrate that the prepared GO dots have a GO structure. From these results, we conclude that the prepared GO dots contain the properties of both GQDs and GO, which will improve the effective utilization of the material for a wide range of applications. PET Studies of GO Dots. Both functionalized and nonfunctionalized carbon dots have been utilized for electron-transfer studies and other applications.54,55 Here, we have utilized the luminescent properties of GO dots for the photoinduced electron-transfer analysis in view of using the GO dots in the energy field. GO dots have been used as an
(1)
where I0 and I are the fluorescence intensities of the GO dots in the absence and presence of viologens, respectively. KSV is the S−V constant and [Q] is the concentration of the respective quencher. The plot between I0/I and [Q] for all three viologens resulted in a linear straight line (Figure S10). From this linear plot, the quenching constants and rate constants were calculated and are listed in Table 1. The calculated kq values are close to the diffusion-controlled rate Table 1. Values of S−V Quenching Constants (KV) and Quenching Rate Constants (kq)
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KSV (M−1)
kq (M−1 s−1)
EV EDV ETV
0.82 × 102 1.19 × 102 1.57 × 102
2.77 × 1010 4.11 × 1010 5.43 × 1010 DOI: 10.1021/acsomega.8b01180 ACS Omega 2018, 3, 9096−9104
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constant limit,58 which is 1010 M−1 s−1. Therefore, it is indicated that dynamic quenching plays the most important role in the quenching of GO dots by viologens. On the other hand, the quenching efficiency of ETV is much higher compared to that of EDV and EV because of the increased viologen units, which are known to enhance the PET. If the quenching is purely due to electron transfer, the photoproducts (cation and anion radicals) must be detected by the transient absorption (TA) technique. Moreover, it is known that the TA signal of the cation radical of viologen can be detected at 600 nm.59 Hence, we have employed the nanosecond TA spectroscopic technique to probe the cation radical of viologen. Figure 4 shows the TA spectra of GO dots with
Figure 5. Comparison of fluorescence quenching efficiency of different nitro aromatics (10 μM) on GO dots (λexi: 360 nm and λemi: 464 nm) in a water medium.
Figure 4. TA spectra of GO dots and GO dots with EV at various delay times. The excitation wavelength is 355 nm. Figure 6. (a) Fluorescence quenching of GO dots with various concentrations of PA (0−10 μM) and (b) GO dot-based test strip, top: drop of GO dots (5 mg/5 mL) and bottom: GO dots with PA (10 μM).
EV at various time delays. The samples were excited at 355 nm. From this figure, it is clear that GO dots alone do not show any transient prominent signals. However, in the presence of EV, the transient signal was observed at 610 nm. We assigned this signal as the cation radical of EV, which is due to the PET reactions between the GO dots and the EV. Hence, we unanimously believed that the observed quenching is due to electron transfer. Detection of Explosives. The finding of explosives is another interesting and significant area of research.60−65 Herein, we have used the fluorescent nature of GO dots for sensing the nitro aromatic chemicals,66 such as TNP or PA, onitrophenol, p-nitrophenol, 2,4-dinitrophenol, nitrobenzene, dinitrobenzene, 2,4-dinitrophenyl hydrazine, and phenol. The fluorescence titration method was performed to analyze the quenching efficiency of nitro compounds. Interestingly, PA showed 80% of quenching efficiency compared with other nitro compounds. These observations reveal that the GO dots are much more perceptive to PA than to the other nitro compounds (Figure 5). Further, to determine the limit of detection by GO dots for the identification of PA, the fluorescence spectra were recorded at different concentrations of PA from 1 to 10 μM (Figure 6a). The quenching of GO dots by PA is quantitatively analyzed by using the S−V equation, I0/I = 1 + KSV [Q]. The S−V plot (Figure S11a) shows a linear curve at lower concentrations and a downward bending curve for higher concentrations (0−10 μM) of PA. A similar type of plot has been reported in the literature.67,68 The S−V constant is calculated directly from the slope by using a linear fitted S−V plot. The nonlinear S−V plots possibly due to both the self-absorption and the energy transfer mechanism69,70 and the S−V constant for the
nonlinear plot were calculated by using an exponential equation I0/I = a exp(k[Q ]) + b
(2)
where a, b, and k are constants. The S−V plot is fitted perfectly with I0/I = 3.38 exp(0.0053[Q]) − 3.43, and the value of the correlation coefficient (R2) is 0.9968, as displayed in Figure S11b. By using the product constants a and k, the S−V constant value for PA was calculated as 1.79 × 104 M−1, and the limit of detection of 92 nM is determined by using 3σ/k, in which σ is the standard deviation and k is the slope. The detection limit comparison of GO dots and other photoluminescent sensors is compiled in Table S1. The lower detection limit explored the practicability of GO dots for the selective detection of PA.71−79 The selective quenching of GO dots by PA is due to the inner filter effect as there is an overlap between the absorption spectra of both GO dots and PA (Figure S12). The test strip-based detection of PA using a thinlayer chromatography plate in view of demonstrating the practical use of GO dots as a chemosensor is also observed. Figure 6b shows the test strip with a drop of GO dots, which is highly emissive in nature under longer ultraviolet light (top); after adding PA, the emission was completely diminished (bottom). The test strip results for the GO dots in the presence of other nitro compounds have also been displayed in Figure S13, which inferred the selective reduction of GO dot emission by PA compared to others. These remarks made it 9100
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biosensor for the selective detection of BR at the microgram level. Overall, the mixed behavior of GQDs and GO renders GO dots an effective usage in diverse applications such as in the environmental-related fields.
clear that the TLC-based test strip can be successfully used for the visual detection of PA in real samples. Detection of Biomolecules. Detection of biomolecules is a fascinating area among the emerging research fields. Various biomolecules with different structures including glucose, sucrose, tryptophan, glycine, glutathione, alanine, BR, and urea were chosen as a typical target material. Fluorescence analysis of GO dots in the presence of the above-mentioned biomolecules was performed individually to find out the quenching efficiency of different biomolecules to GO dot emission. Among them, BR showed 50% quenching efficiency. This observation clearly revealed that the GO dots are more sensitive to BR than the other biomolecules (Figure 7).
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MATERIALS AND METHODS Materials. CA was procured from Fisher Scientific. Other reagents were of analytical grade and bought from LOBA Chemicals (India). Nitro compounds and biomolecules were purchased from Merck. Milli-Q water was used as a medium in spectral analysis. Characterization Techniques. HR-SEM measurement was done for the dried GO dot sample with the use of Nova NanoSEM 600 from FEI Company, Netherlands, and elemental analysis was done using an EDAX system. The average size of the particles was probed by HRTEM (JEOL JEM-2100) with an accelerating voltage of 120 kV. The specimen for the TEM analysis was prepared by drop-coating a GQD solution on the carbon-coated copper grid, which was further completely dried by slow evaporation of the solvent. The hydrodynamic size of the particles in medium, dispersibility, and surface charge were observed by the dynamic light scattering method by using a Malvern DLS instrument. FT-IR spectral analysis was done with the help of a JASCO FT-IR ATR 6300 spectrometer at ambient temperature in the spectral range of 4000−400 cm−1. Raman spectrum was recorded using a high-resolution Renishaw Raman microscope using a He−Ne laser of 18 mW at 633 nm. NMR spectra were recorded on a Bruker instrument operating at 400 MHz for 1H and 100 MHz for 13C, and the details were collected at ambient temperature and the spectral data were calibrated to an internal tetramethylsilane standard. UV−vis absorption spectral measurements were obtained using a PerkinElmer Lamda 25 spectrophotometer. The emission nature and steady-state fluorescence quenching analysis were done using a PerkinElmer LS 45 spectrofluorometer. The slit width for both excitation and emission was fixed as 5 nm, and the rate of scanning was fixed as 200 nm min−1. Excited-state decays were measured in a time-correlated single-photon counting system using a fluorocube fluorescence lifetime spectrometer (JOBIN-VYON), and the sample was initially excited at 370 nm. Data analysis was carried out by the software provided by IBH (DAS-6), based on the deconvolution procedure using a nonlinear least-squares technique, and the excellence of the fit was determined with the value of χ2 < 1.2. Synthetic Procedure. The GO dots were derived from CA via direct pyrolysis, a facile, low-cost, and bottom-up method based on the reported protocol9 with some modifications. The typical procedure followed is as follows: 5 g of the precursor was taken in a dried beaker and heated at 200 °C. It is noteworthy to mention that when the heating was initiated, exactly at the melting point of CA, we obtained a colorless solution, which changed to an orange solution beyond its melting point. Further, continued heating for 1 h resulted in the conversion of the orange solution to a dark brownish colloid, indicating the formation of a carbon-based material. Further, 1 M NaOH was added dropwise into the solution under continuous stirring. Subsequently, the resulted solution was neutralized by dropwise addition of dilute HCl, which turned the solution color into orange. Finally, the reaction products were centrifuged for 30 min at 3000 rpm in order to eliminate the black insoluble carbonaceous materials,
Figure 7. Comparison of fluorescence quenching efficiency of different biomolecules (10 μM) on GO dots (λexi: 360 nm and λemi: 464 nm) in a phosphate buffer medium.
Consequently, in order to find out the detection limit, the fluorescence quenching measurement was carried out with different concentrations of BR as projected in Figure S14a, which clearly indicates the linear quenching of GO dots by BR in the concentration range of 0−10 μM. The linear dynamic type of quenching was determined by the S−V plot as shown in Figure S14b, and the inner filter effect is suggested for the mechanism of quenching, which was due to the absorption spectral overlap between the GO dots and BR (Figure S15). Further, the limit of detection of BR was calculated to be 0.52 μg/mL, according to a signal-to-noise ratio of S/N = 3. Moreover, the value of limit of detection was much lesser than the limit of BR in blood plasma (19.80 mg/mL in a newborn and 1.19 × 10−2 mg/mL in adults).47 The detection limit comparison for BR by other photoluminescent sensors is compiled in Table S2. Hence, these experimental results clearly illustrate that the GO dots could be applied as an efficient fluorescent bioprobe for the selective and sensitive detection of BR.
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CONCLUSIONS In summary, a successful synthesis of GO dots by fine-tuning the carbonization extent of CA via a facile as well as a singlestep pyrolysis method is reported. The prepared GO dots exhibited the mixed properties of GQDs as well as GO, and it was thoroughly characterized by pivotal techniques. PET studies of GO dots with viologens explored the effective utilization of GO dots in the energy field as a shuttling mediator. The excellent luminescent properties of GO dots were used for choosy and quantitative detection of PA in a nanomolar range and have also been demonstrated via the test strip method. Furthermore, GO dots have been used as a 9101
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and the transparent orange supernatant was separated out. Further, the orange solution was passed through a disposable cellulose membrane filter with a pore size of 0.2 μm followed by extensive dialysis done against Millipore water using a dialysis tube (15 000 molecular weight cutoff) for 48 h to remove the small fluorescent molecules (dialysis water was changed at 8 h intervals). After dialysis, the clear orange solution containing fluorescent carbon dots was concentrated by distillation under reduced pressure and dried out in a vacuum oven. The obtained solid product exhibited high photostability (Figure S16a), and it is water-dispersible in nature. The storage stability of the GO dots has also been ensured by storing the solution at 4 °C, which is stable for more than 3 months (Figures S16b and S17). A product yield of 65% is obtained from the above procedure. The images of CA and synthesized GO dots are shown in Scheme S2. It is observed that the precursor (CA) is a colorless and crystalline solid compound, and after pyrolysis, it turned to a browncolored crystalline solid product. The proposed mechanism for the formation of GO dots from CA is the carbonization of CA during pyrolysis to some extent, which leads to the generation of graphite structures via removal of water molecules as well as hydrogen bonding between the carbonyl and OH groups. The extended carbonization of CA leads to the formation of both GO sheets and GQDs, which are together called as GO dots (Scheme S4).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01180. Structure of viologen derivatives and images of the precursor as well as GO dots prepared; FT-IR spectrum, HR-SEM images, zeta potential, 1H NMR, EDAX data, Raman spectra, and absorption and emission spectra of GO dots with viologen derivatives and BR (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (M.A.J.). *E-mail:
[email protected],
[email protected] (A.K.). ORCID
Murugavel Kathiresan: 0000-0002-1208-5879 Arunkumar Kathiravan: 0000-0001-6131-460X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS V.S. thanks the Department of Chemistry and BSACIST for the Junior Research Fellowship. M.A.J. thanks Nano-IITM and SAIF, IIT Madras, for assistance with NMR and HR-SEM facilities. A.K. acknowledges the National Centre for Ultrafast Processes (NCUFP), University of Madras.
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DOI: 10.1021/acsomega.8b01180 ACS Omega 2018, 3, 9096−9104
