Synthesis of Excitation Independent Highly Luminescent Graphene

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Synthesis of Excitation Independent Highly Luminescent Graphene Quantum Dots through Perchloric Acid Oxidation Susmita Maiti, Somashree Kundu, Chandra Nath Roy, Tushar Kanti Das, and Abhijit Saha Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02611 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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Synthesis of Excitation Independent Highly Luminescent Graphene Quantum Dots through Perchloric Acid Oxidation Susmita Maiti, Somashree Kundu, Chandra Nath Roy, Tushar Kanti Das and Abhijit Saha*

UGC-DAE Consortium for Scientific Research, Kolkata Centre, III/LB-8 Bidhannagar, Kolkata 700098, India; E-mail: [email protected]; fax: +91-33-2335 7008

KEYWORDS Graphene quantum dots. Quantum yield. Excitation Independent

photoluminescence. Delayed fluorescence, anisotropy.

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Abstract We demonstrate a facile liquid phase exfoliation method by only using perchloric acid to synthesize graphene quantum dots (GQDs) having excitation independent strong emission with quantum yield of about 14%. The proposed simplified synthesis strategy can help in overcoming the limitations of existing aqueous routes which produce GQDs with excitation dependent emission and of low quantum efficiency. Photoluminescence (PL) properties of GQDs have been studied in detail to understand the origin of emission. As-synthesized GQDs show excitation independent photoluminesce (PL) which suggests that the synthesized materials do not have any significant defects. Spectral analysis suggests that the PL emission of the well-defined GQDs originates mainly from the peripheral functional groups conjugated with carbon backbone planes. We also demonstrate a relatively longer PL life time (average life time of about 10 ns) of the synthesized GQDs determined by time correlated single photon counting (TCSPC) measurement and this high lifetime suggests that the synthesized GQDs may be suitable for biomacromolecular probing. In addition, as-synthesized GQDs interestingly show delayed fluorescence and steady state anisotropy, which make the material appropriate candidate for application in sensing, bioimaging of cells and organism.


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Introduction In recent year, graphene, consisting of two dimensional layer of carbon organized in a honeycomb like crystal lattice has attracted tremendous attention in scientific research community because of its many novel properties like, high mechanical strength, high mobility and various applications in optics and electronics.1-5 For being a zero band gap semiconductor it seems unlikely to observe photoluminescence (PL) from graphene and this limits its application in the area of optoelectronics. In comparison with two dimensional zero band gap of graphene sheet, zero dimensional, graphene quantum dots (GQDs) are promising photoluminescent graphene materials in the visible region encompassing wide applications, like photovoltaics, bioimaging, light-emitting diodes, and sensors.6-11 Thus, GQD is recognized as an important member of fluorescent based carbon nanomaterials12 which contain fullerenes,13 nanotubes,14 nanodiamond15 and carbon nanoparticles16,17 due to its strong quantum confinement and edge effects. In contrast to the normal carbon dots which are quasi-spherical, GQDs having high aspect ratios of the height to the lateral size possess significantly greater crystallinity.6,18 All these features make GQDs structurally distinctive and more advantageous than common carbon dots. Having enormous availability of π-π conjugated network, exceptional biocompatibility or low toxicity, graphene quantum dots attract more attention than the conventional semiconductor quantum dots for the applications in different fields like photocatalysis, upconversion photoluminescence, bioimaging, and metal ion detection.18-22 In addition, graphene quantum dots are useful in the development of research in electrochemical biosensors, drug delivery, biomedicine and energy conversion because of available π-π conjugated network with peripheral functionalizable surface groups and high surface area. 23,24


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Synthesis of GQDs is usually categorized under top-down or bottom-up method. The top -down approach involves cutting of graphene sheets or graphite crystallites into GQDs by chemical oxidation, mechanical grinding, electrochemical approach etc.25,26 On the other hand, bottom-up approach involves oxidative condensation of aryl group and ruthenium catalyzed cage opening of fullerene C60 molecules,27 or chemical synthesis from citric acid or other organic groups.28,29 The former involves simple and cost effective equipments while the latter requires sophisticated equipment, extremely expensive raw materials and products of low quantum yield. In the “top-down” method, GQDs have been fabricated by cutting sp2 clusters in graphene sheet by acidic oxidation, hydrothermal process, electrochemical oxidation and so on. Peng et al prepared GQDs by oxidation of carbon fibers with a mixture of concentrated HNO3 and H2SO4 and tunable luminescence from blue to yellow was observed as the reaction temperature was varied from 80 to 120 °C.30 Pan et al. developed hydrothermal route for synthesizing GQDs from graphene sheet.26 An electrochemical approach for direct preparation of functional GQDs with a uniform size of 3-5 nm as potential electron acceptors for photovoltaics was presented by Li and co-workers.25 Shen et al. proposed a hydrazine hydrate reduction of graphene oxide method for the fabrication of GQDs with upconverted emission.31 However, GQDs produced through these methods exhibit excitation wavelength dependent emission having low photoluminescence quantum yield (PLQY). There are also a very few literatures which show excitation independent PL of GQDs synthesized through “top-down” methods, like microfludization of graphite,32 ultrasonication of graphene following the treatment with mixed acid for fabrication of GQDs,19 electrochemical method,33 sonochemical process involving organic solvent assisted liquid phase exfoliation of graphite34, etc. However, these experimental methods are complex in nature to be useful for bulk production and moreover, these produce


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materials of low quantum yield. Thus, the major challenge remained is how to produce defects free GQDs and to improve its luminescence characteristics and quantum yield. Because, highly defective structures with numerous sp3 and basal-plane vacancies induce poor optical properties, controlled creation of graphene derivatives with low defects, become increasingly meaningful in various biologhical applications.35 There are also reports of defects selective synthesis of GQDs by ultrasonic assisted liquid phase exfoliation technique.36 But, the main drawbacks are excitation wavelength dependent PL and low quantum yield of the synthesized material. Among the existing methods, the acid treatment technique offers simplified and less expensive method. Herein, we report a modified acid treatment method involving graphene oxide and perchloric acid (HClO4) for synthesizing excitation independent photoluminescent GQDs with significantly low defects. We also investigated the dependence on concentration of precursor GO as well as the concentration of acid in the synthesis of GQDs for developing optimal conditions to produce good quality GQDs. The present work demonstrates that perchloric acid oxidation can generate GQDshaving excitation independent PL with high quantum efficiency. Being more stronger than mixed acid, i.e. sulfuric and nitric acid use of perchloric acid (pKa ≈ -10) is advantageous than mixed acid. It provides strong acidity with minimal interference because perchlorate is weakly nucleophilic which helps in its direct use in further biological systems. GQDs were characterized by Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), etc. As-synthesized GQDs possess many interesting optical properties, such as, strong excitation independent fluorescence with high PL quantum yield, delayed fluorescence at room temperature, steady state anisotropy etc.


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Experimental Section Materials Natural graphite powder (300 mesh) was purchased from Alfa Aesar and used without furtherpurification. Perchloric acid (70% v/v), sodium hydroxide, hydrochloric acid and Quinine Sulphate (Dihydrate) were purchased from Merck, India. All chemicals are of analytical grade and deionized water was used in the experiment.

Synthesis of Graphene Quantum Dots At first, graphite oxide was prepared by oxidation of natural graphite using sulphuric acid (98% v/v), potassium permanganate and sodium nitrite following Hummers method.37 Graphite oxide thus obtained was further exfoliated via sonication to get graphene oxide (GO). The details of the synthesis procedure and characterization of GO were reported in our previous work.38 10 mL of as-synthesized GO solution of (1, 2, 5 mg mL-1) were mixed with concentrated perchloric acid (HClO4) and stirred for half an hour at room temperature. The mixed solution was subsequently taken in a 100 mL round bottle flask fitted with a reflux condenser and heated at 100OC for 24 hours. This solution was then cooled to room temperature and sonicated for 1 hour. After that, this acidic solution was neutralized to pH 7.0 with sodium hydroxide solution and the colour turned to deep brown indicating the formation of GQDs. The deep brown solution of GQDs was filtered to remove large particles and then dialyzed through the membrane (molecular weight cutoff 6000 Dalton) to remove electrolytes. Finally, the dialyzed solutions of GQDs were characterized by TEM, XPS, XRD, Raman spectroscopy, etc. In order to look into the effects of acid concentration, the same method was followed by keeping precursor concentration or [GO]


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fixed at 1 mg mL-1 and varying the acid concentration as 6.27, 8.36 or 11.15 mol L-1, respectively. Methods TEM measurement was carried out on a (JEOL, JEM-2100) operated at an acceleration voltage of 200 kV. A drop of aqueous solution of GQDs was placed on a carbon-coated copper grid of 400 mesh and dried overnight before putting into TEM sample chamber. X-ray diffraction measurement was performed using Bruker D8 Advance X-ray diffractometer operated at a current of 40 milli Amp and 40 kV voltage. The scan rate was 2o/minute in the 2θ range of 10–80o during data accumulation. The Cu Kα (λ = 0.1546 nm) was used as the radiation source for the experimental measurements. X-ray photoelectron spectroscopy (XPS) ( M/s Specs, Germany ) of GO and GQD system was performed using a focused monochromatized Al Kα X-ray source (1486.74 eV). Raman spectral measurement was recorded using a Lab Ram HR 800 (Horiba Jobin Yvon) spectrometer. The instrument acquired data over a range of 200 cm−1 to 3500 cm−1 with 5 sec exposure time. The laser power was 17 mW and the operating wavelength of the He-Ne laser was 632.8 nm. Spectral detection was obtained through the use of a air cooled charge-coupled device (CCD). The samples for Raman spectral measurements were prepared in the form of film on glass and were excited with the laser. FTIR spectroscopic measurements of GQDs were recorded using a FTIR spectrometer (Perkin Elmer, Spectra GX). Samples for FTIR measurements were prepared in the form of pellets by mixing 200 mg of IR spectroscopic grade potassium bromide with 2 mg of dried sample. The spectra were recorded in transmission mode with an average of 20 scans with a resolution of 4 cm-1.


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The absorbance spectra of as-synthesized graphene quantum dot were recorded using a UV-Vis-NIR spectrophotometer (Perkin Elmer, Lambda 950). The photoluminescence (PL) mesurements were performed by Perkin Elmer (LS55) luminescence spectrometer. Photoluminescence quantum yield (PLQY) of synthesized GQDs, was measured employing quinine sulfate solution in 0.1 M H2SO4 as standard. To obtain the integrated intensity of GQDs and standard sample, corrected emission spectra of both quinine sulfate and GQDs were taken. Then, PLQY of synthesized GQDs was calculated by using the relation (eq. 1)

=

×

×

(1)

where, φ is the quantum yield, Ι, the measured integrated emission intensity, η, the refractive index of the solvent, and A, the absorbance. The subscript "St" refers to standard with known quantum yield and "x" for the sample. In order to minimize re-absorption effects, absorbance was kept below 0.10 at the excitation wavelength of 320 nm. Fluorescence lifetimes of GQDs were determined by using a time-correlated singlephoton-counting (TCSPC) spectrometer (Fluorolog TCSPC Horiba Jobin Yovon) using nanosecond diode laser of 373 nm (Horiba scientific, DD-375L). The decay kinetics were monitored at an emission wavelength of 437 nm. The data were analyzed by DAS-6 decay analysis software. The fluorescence decay curves were analyzed by a multi-exponential fitting program considering reduced chi-square value. pH measurement was done by pH cum ion meter (Jenway-3345). Sonication was carried out with Ultrasonic Homogenizer (Model: U650) Takashi, Japan at 25 kHz by using 2 mm probe.


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Results and Discussions Characterization of synthesized GQDs The as-synthesized GQDs was deep brown in colour and highly soluble in water and other organic solvents, like dimethyl sulfoxide (DMSO), isopropanol, dimethyl formamide (DMF), etc. The GQDs solution remained homogeneous at room temperature about 3 months without no further precipitation and no change in PL intensity. As-synthesized GQDs were characterized through many experimenatal techniques including Transmission Electron Microscopy (TEM), XRD, Raman spectroscopy, XPS etc. Figure 1a shows typical TEM image of GQDs. As we can see that particles are spherical in nature and are of average size about 5.6 nm as obtained from the plot (Figure 1b) of size distribution.26,33 Figure 1c displays high resolution TEM image of individual GQDs. The distinct lattice fringes of GQDs corresponds to higher crystallinity and the lattice spacing of 0.22 nm represents (100) plane of graphene. The crystal plane calculated from selected area electron diffraction pattern (SAED) (Figure 1d) matches with (202) and (110) planes of Hexagonal GQDs (JCPDS PDF26-1075). Figure S1 presents the TEM image of precursor GO and Figure S2 shows analysis of Electron Dispersive Spectrum (EDS) of GQDs, which illustrates the presence of carbon and oxygen content and their atomic percentages are 41% and 33%, respectively.39


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Figure 1. a) TEM image of as-synthesized GQDs, b) diameter distribution of GQDs, c) HRTEM image of GQDs, d) SAED pattern of GQDs.

Figure 2 shows X-ray diffraction pattern of GQDs. A typical broad peak near 2θ value of 23.160 0

was observed for GQDs corresponding to layer to layer distance or d-spacing of 3.84 A of (002) plane of graphite. There are another two peaks at 31.80 and 45.80 which correspond to (100) and (102) planes of hexagonal carbon, respectively (JCPDS No. 41-1487). In comparison to starting material GO, a significant peak shift occurs from 9.830 to 23.160 and decrease in dspacing clearly indicates more compact two dimensional structure.33,25


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Figure 2. XRD pattern of GQDs. Raman spectrum of GQDs (Figure 3) shows two characteristic peaks at 1336 and 1600 cm−1, which are assigned to the D band (related to the disorder induced phonon mode of vibrations of sp3 carbon atoms) and G band (associated with the first-order scattering of E2g mode for sp2 carbon lattice of graphitic domain), respectively and these are similar to the hydrothermally synthesized GQDs.26 The ID/IG intensity ratio of GQDs is 1.3 which is about 41% lower than that of the graphene oxide 2.2, indicating a decrease in disorderness in the synthesized GQDs.

Figure 3. Raman spectra of synthesized GO and GQDs. 11 ACS Paragon Plus Environment

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X-ray Photoelectron spectroscopy (XPS) measurements were carried out to probe the chemical compositions of GQDs and also change in chemical environment of GQDs from the starting material GO. Figure 4 (a,b,c) demonstrate the survey spectra, corresponding deconvoluted C 1s spectra and N 1s XPS spectra of GO. In addition, survey spectra of GQDs, corresponding deconvoluted C 1s and Cl 2p XPS spectra are shown in Figure 4 (d,e,f). The survey spectra of both GO and GQDs show two dominant peaks for C1s at ca. 285.4 eV and for O 1s at ca. 534.8 eV, and small amount of N 1s for GO at ca. 400 eV, but there are no detectable amount of N 1s for GQDs (Figure 4 a,d).30,33 In contrast, the survey spectra of GQDs show the presence of Cl 2p peak at ca. 203.5 eV which is expected to originate from attachment of chlorine atom to the carbon skeleton from perchlorate ion used in the synthesis process.40 The high resolution C 1s spectrum of GO (Figure 5b) can be deconvoluted into five different peaks which correspond to five different functional groups of carbon e.g., C-OH (285.6 eV), C–O epoxy/ether groups (286.6 eV), carbonyl carbon (287.8 eV) and O-C=O (289.6 eV). The deconvoluted C 1s spectrum of GQDs also shows presence of oxygen functional group after further oxidation e.g., carbonyl carbon (288.8 eV), O-C=O (290.3 eV) and another one component was observed at ca. 292.2 eV for C-Cl (Figure 5e). Figure 5c and f show higher resolution deconvoluted spectra of N 1s of GO and Cl 2p of GQDs. The N 1s spectrum of GO can be divided into two different peaks, a lower energy peak (400.3 eV) for graphitic N atoms and a higher energy peak (402.5 eV) which is well attributed to pyrazole-N atoms. Figure 5f shows well fitted two peaks one at ca. 203 eV of Cl 2p1/2 and another Cl 2p3/2 at ca. 204.9 eV. These observed results indicate presence of functional groups of high oxygen content similar to GO, such as, C-O, C=O, O-C=O and unlike GO, a new functional group, C-Cl is generated in the synthesized GQDs.


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Figure 4. (a) XPS survey spectra of GO, (b) deconvoluted higher resolution C 1s spectra of GO, (c) N 1s XPS spectrum of GO, (d) survey spectra of GQDs, (e) deconvoluted higher resolution C 1s spectra of GQDs and (f) Cl 2p spectrum of GQDs.

The Fourier transformed infra-red spectra of GO and GQDs are shown in Figure 5. The stretching vibrations of C-OH bond at 3505 cm-1, sp2 hybridised C=C at 1646 cm-1, carboxyl group at 1404 cm-1, C-O peaks at 1101 cm-1, C-Cl at 632 cm-1 are present in GQDs.41-43 All peaks are shifted from the precursor GO. The carbonyl peaks are weakened in GQDs in comparison with precursor GO.


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Figure 5. FTIR spectra of GO and GQDs.

UV- Vis absorption spectroscopy Figure 6, a and b illustrate absorption spectra of GQDs synthesized with different concentrations of added acid keeping concentration of precursor GO fixed and conversely, varying of concentrations of GO maintaining acid concentration fixed. All the GQDs synthesized with fixed -1

concentration of GO (1 mg mL ) exhibit absorption shoulders at 265, 281, 275 nm when concentration of acid were used as 6.27, 8.36, 11.15 mol L-1, respectively. Absorption band in the range of 290-300 nm indicates n- to -π* transition of functional group C=O, which is blue shifted in synthesized GQDs (Figure 6a).44 The difference in absorption profile could be due to varying extent of oxidation with variation of concentration of perchloric acid. Thus, it is expected to have higher oxygen content in the materials with increasing concentration of acid. -1

Subsequently, with variation of concentration of precursor GO from 1 to 5 mg mL for a given acid concentration of 8.4 mol L-1, absorption hump arises around 280 nm (Figure 6b) which is in 14 ACS Paragon Plus Environment

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well agreement to the previous report.45,46 The absorption bands around 276, 281 and 280 nm are attributed to the n- to -π* transition of the C=O bond from 290–300 nm. At relatively higher concentration of GO (5 mg mL-1) employed in the synthesis, two additional bands appear at 231 nm and 317 nm which are identical to the absorption behavior of precursor GO.47 The absorption hump at 231 nm is assigned to π- to -π* transition of C=C of the oxidized aromatic structure.47 Absorption humps at 231 and 317 nm observed (Figure 6b) at higher concentration of precursor GO may be attributed to the presence of unreacted GO. Thus, it appears that 2-3 mg mL-1 is the optimal precursor concentration with perchloric acid concentration of 8.3 mol L-1 for obtaining good quality GQDs.

Figure 6. UV-Vis absorption spectra of GQD at (a) varied concentrations of perchloric acid keeping GO concentration fixed and (b) varied concentrations of precursor GO with fixed acid concentration. Photoluminescence Spectroscopy To further explore the optical properties of as-synthesized GQDs, a detailed investigation of fluorescence at different excitation wavelength has been carried out. In most of the carbon


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nanoparticles, PL emission is excitation wavelength dependent. However, it is important to note here that GQDs synthesized through the present route show an excitation-independent PL. When the excitation wavelength changes from 260 to 350 nm, the intensity of PL spectra increased to maximum then further decreased but peak position remained unshifted at ca. 437 nm (Figure 7a). This feature is different from other carbon quantum dots.19 It has been revealed that the π*- to -n transition of carbonyl or carboxylic groups connected to carbon nanotubes and other fullerene carbon nanostructured materials produce PL emission around 430 nm with excitation of 320 nm.48 Also, the emission at 437 nm is generated not only because of π*- to -n transition of carbonyl or carboxylic groups of GQDs but is partially contributed from conjugated carbon skeleton.49 There are several mechanisms which illustrate the origin of the excitation wavelength dependence of the PL property of GQDs.50-53 The most general explanation concerns quantum size, zigzag edge sites, recombination of localized electron−hole pairs and defect effects. The first three contributions regarded as intrinsic state emissions result in excitation independent PL. In contrast, majority of GQDs synthesized so far through different approaches show excitation dependent PL emission, which are, in general, attributed to presence of defects states. 49 Hence, it may be inferred that GQDs synthesized through the present HClO4 mediated oxidation process do not have any significant defect states.32 The synthesized GQDs possess an important and desired optical characteristics which make it very suitable for various applications. Figure 7b demonstrates PL behavior of GQDs at variable GO concentration by keeping acid concentration constant. The results show that PL intensity increases with increasing concentration of GO and this may be due to formation of increasing number of GQDs. The results also illustrate the Full width at half maximum (FWHM) of the PL spectrum of GQDs at lower acid concentration is broad which indicates large size distribution of the as-synthesized


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material. But increase in the precursor concentrations of GO results in decrease in FWHM of the PL spectra of GQDs. Interestingly, with increasing acid concentration, the fluorescence intensity of GQDs decrease (Figure 7b, inset). The possible explanation is that higher acid concentration results in formation of more complexes between free zig-zag sites and H+ which may cause breaking of emmisive trap of triplet carbene and leads to quenching of PL (inset of Figure 7b).54 The bright blue fluorescence is quantified in terms of fluorescence quantum yield at 320 nm excitation wavelength taking quinine sulphate as reference. The estimated quantum yield is 13.6% which is considerably higher than that of fluorescent graphene and other GQDs prepared by mixed acid or other methods so far.30,39

Figure 7. Photoluminescence spectra of GQDs at (a) various excitation wavelength and (b) different concentrations of GO. Fig. 7b: inset shows PL spectra of GQDs at various concentrations of perchloric acid.

The photoluminescence excitation spectrum (PLE) (Figure 8a) shows two peaks at 250 nm and 325 nm. These two PLE peaks demonstrate that the observed luminescence occurs not only from


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the common π*- to -π transitions but also correlated to two new transitions at 250 and 325 nm. The origin of luminescence of GQDs is more similar to that of triplet carbene (σ1 π1) at its zigzag edges corresponding to the two electronic transitions observed from two PLE peaks at 250 nm (5.0 eV) and 325 nm (3.82 eV).26 Thus, the electronic transition can be regarded as the transition from σ and π orbitals and occurs from the HOMO to LUMO as shown in figure 8b. The energy difference (∂E) is 1.18 eV ( 1.5 eV for triplet carbene) which is reasonable value for observed luminescence as described by Hoffman.55

Figure 8. (a) PLE spectra of GQDs at the emission at 430 nm and (b) typical electronic transitions observed in carbene like states of GQDs.

Time Correlated Single photon Counting (TCSPC) measurements Steady state luminescence provides us with valuable information about population of different energy states. TCSPC measurement can provide important information about lifetime of excited electron which has importance in many applications, such as, macromolecular probing, etc. Figure 9 illustrates time resolved luminescence decay of GQDs at 437 nm when excited at 373 nm. The decay profiles are fitted following the eq. 2. 18 ACS Paragon Plus Environment

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= ∑ /

(2)

where, τi is the lifetime of the different components and the pre-exponential factor, Bi, is a measure of the contribution of each component to the total decay. The average lifetime of fluorescence decay, τav is calculated using the following relation (eq. 3).56 The lifetime data of GQDs are well fitted in tri-exponential function.

!

= ∑ " ⁄∑ (3)

Figure 9. Emission decay profile of GQDs at (a) different concentrations of precursor GO with perchloric acid concentration of 8.36 mol L-1 and (b) at various concentrations of perchloric acid with concentration of GO as 1 mg mL-1. Figure 9 compares between the decay profiles of GQDs synthesized at variable concentrations of starting GO, by keeping concentratition of perchloric acid fixed and conversely at different acid concentrations by keeping fixed concentration of precursor GO. The three lifetime components for the first are 1.93, 8.23 and 0.14 ns with relative amplitudes of 34.88, 37.02 and 28.11%, respectively. About 77% increase in life time of the relatively short lived component from 0.14 to 0.62 ns suggests increase in lifetime of aggregated species of –COOH riched GQDs formed in the excited state at different concentrations of starting materials.45 The


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long-lived component shows that the lifetime of aggregated species of –OH riched GQDs increases from 8.2 to 9.2 ns. There is almost no change in average lifetime (approximately 7 ns). The increasing acid concentration results in about 14% decrease in short lifetime componentfrom 0.43 to 0.37 ns suggesting decrease of aggregated species in the media or increase of monomeric species in acidic media. It may be assumed that higher acid concentrations lead to protonation of functional groups which results in breaking of conjugated structure. The relative amplitudes of short lifetime components decrease due to removal of monomeric species and formation of aggregated species of –COOH riched GQDs. Decrease in value of long-lived components from 14 to 8.4 ns suggests decrease in aggregated species with increasing acid concentration. However, the high average lifetime of about 10 ns suggests that the synthesized GQDs are appropriate for various optoelectronic and biological aplications. Phosphorescence Measurement Room temperature phosphorescent (RTP) material shows tremendous application in sensing, bioimaging of cells and organism.57 Although fluorescence from GQDs is well documented but the phosphorescence behavior has not been reported earlier so far except in one literature.34 Interestingly, we have found phosphorescence or delayed fluorescence in the synthesized GQDs when excited with uv photons at room temperature. Figure 10a shows phosphorescence spectra having emission peak around 430 nm with excitation of 320 nm, which coincides with the normal fluroscence spectra of GQDs. Hence, this phosphorescence may be considered as delayed fluorescence.35 Phosphorescence and delayed fluorescence often observed in organic molecules or molecules which are held in a rigid environment to reduce collisional de-excitations. In case of fluorescence, the electronic transition occurs from S1- to -S0. But, in case of phosphorescence inter-system crossing (ICS) occurs from singlet (S1) to triplet (T1) state and then from (T1) to


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ground state (S0) which is spin forbidden. Most of the phosphorescent materials, contain heavy atoms to enhance spin-orbit coupling which can assist intersystem crossing. But the heavy metals are toxic, expensive and unstable. In this context, organic compounds are very desirable, but the problem is weak spin–orbit coupling to facilitate intersystem crossing. But in GQDs the energy gap between singlet and triplet state is low because of having conjugated π-structure, so the spin– orbit coupling is efficient, and then the intersystem crossing is efficient as well.58 Furthermore, the presence of heavy atoms is known to promote decay via phosphorescence. For this reason πconjugated polymers are considered as nanophosphorescent. With increasing excitation wavelength, phosphorescence spectra also shift to higher wavelength creating excitation wavelength dependent emission. To know the origin of phosphorescence, we look into the phosphorescent excitation spectra with emission at 430 nm which has a broad band at 250-340 nm (Figure 10b). This is assigned as the absorption of C=O bond and so, the origin of RTP is carbonyl moieties of π-conjugated aromatic structure of GQDs.59

Figure 10. (a) Phosphorescence spectra of GQDs at different excitation wavelength and (b) the phosphorescence excitation spectra with emission at 430 nm. 21 ACS Paragon Plus Environment

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Steady State Anisotropy Measurement The study of emission anisotropy for polymers and biological macromolecules are very popular. A linearly polarized light excites those molecules whose transition dipole moment is parallel to the light field and generate polarized emission. It is well known that fluorescent anisotropy is absent in spherical nanoparticles or semiconductor quantum dot. Being quasi-spherical in shape, our synthesized GQDs show positive anisotropy value. So, the structural anisotropy should originate from strong electronic polarization in the excited states and this can allow photoselection by polarized light of fluorophores in a particular orientation resulting in polarized emission.60 The anisotropy ($) (eq. 4) illstrates the fluorescence polarization defined by the following relation

$ =

∥ &

(4)

∥ '"&

where (∥ and () are the intensities of polarized light parallel and perpendicular to the direction of polarization of the laser excitation. Figure 11 shows different anisotropy values against different

Figure 11. Plot of steady state anisotropy versus excitation wavelength.


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excitation wavelength. The anisotropy value initially increases with excitation wavelength reaching maximum at 350 nm followed by a decrease till 400 nm observed. The positive anisotropy observed in the GQDs suggests that the excited state dipole is oriented in a fixed direction. So, GQDs can be possible good candidates for biophysical probing in the dynamics of large bio-macromolecular structures.

Conclusion In summary, we have developed a simple one step single acid, namely perchloric acid, mediated synthesis method to fabricate nanofunctionalised GQDs. Being more stronger than mixed acid it is safer to handle in comparison with mixed acid and perchlorate ion has no interference in further biological applications. We also investigated the dependence on concentration of precursor GO as well as the concentration of acid in the synthesis of GQDs for developing optimal conditions for producing good quality GQDs. As-synthesized GQDs show excitation independent PL which suggests that the synthesized materials do not have any significant defects states. This result is in contrast to PL spectra of other luminescent carbon nanoparticles, such as carbon quantum dots (CQDs).We also observed excitation dependent delayed fluroscence and presence of steady state anisotropy. The time correlated single photon counting (TCSPC) measurement demonstrates a relatively longer PL life time of the synthesized GQDs, which makes it suitable candidate for biomacromolecular probing.

Associated Content Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis procedure and TEM image of graphene oxide, EDS of GQDs, Raman spectral characterization of GQDs. Author Information Corresponding Author E-mail: [email protected]; fax: +91-33-2335 7008 Orcid Abhijit Saha: 0000-0002-0278-1600 Notes The authors declare no competing financial interest.

Acknowledgements The authors are thankful to Prof. N. Kalarikkal of Mahatma Gandhi University for providing with the Transmission Electron Microscopy (TEM) facility and Dr. Shamima Hussain of UGCDAE CSR, Kalpakkam Node for XPS measurements. One of the authors (S. Maiti) is grateful to DST, Govt. of India, for the award of the INSPIRE Fellowship. The author S. Kundu is thankful to the University Grants Commission for the NET fellowship. One of the authors, T. K. Das, is grateful to CSIR for the NET fellowship.


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TOC Graphic

Synthesis of Excitation Independent Highly Luminescent Graphene Quantum Dots through Perchloric Acid Oxidation Susmita Maiti, Somashree Kundu, Chandra Nath Roy, Tushar Kanti Das and Abhijit Saha* UGC-DAE Consortium for Scientific Research, Kolkata Centre, III/LB-8 Bidhannagar, Kolkata 700098, India; E-mail: [email protected]; fax: +91-33-2335 7008

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Synthesis of Excitation Independent Highly Luminescent Graphene

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