Green Synthesis of Multifunctionalized, Nitrogen-Doped, Highly

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Green synthesis of multi-functionalized, nitrogen-doped, highly fluorescent carbon dots from waste expanded polystyrene and its application in the fluorimetric detection of Au ions in aqueous media 3+

Vadivel Ramanan, Bommana Siddaiah, Kaviyarasan Raji, and Perumal Ramamurthy ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02852 • Publication Date (Web): 18 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

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ACS Sustainable Chemistry & Engineering

Green synthesis of multi-functionalized, nitrogendoped, highly fluorescent carbon dots from waste expanded polystyrene and its application in the fluorimetric detection of Au3+ ions in aqueous media Vadivel Ramanan,a Bommana Siddaiah,b Kaviyarasan Raji,a and Perumal Ramamurthy*a a

National Centre for Ultrafast Processes, University of Madras, Taramani Campus, Chennai -

600113, Tamilnadu, India. b

Department of Nanotechnology, Acharya Nagarjuna University, Nagarjuna Nagar, Guntur -

522510, Andhra Pradesh, India.

ABSTRACT: Synthesis of highly luminescent carbon dots (CDs) from waste materials gains much attention in the current scenario. We have converted waste expanded polystyrene (EPS), a non-biodegradable environmental pollutant into multi-functionalized fluorescent CDs. This can be a good scaling up approach for the large-scale synthesis of nitrogen-doped CDs with high photoluminescence (PL) quantum yield (QY) of ~20 %. The as prepared CDs exhibit excellent water-solubility and a longer PL lifetime (in ns). It also possesses excellent photostability, low cytotoxicity and possessing stable luminescence QY at different solution environments. Selective and sensitive

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detection of Au3+ ions is demonstrated using these CDs as fluorescence probes and LOD of 53 nM is achieved. A detail investigation revealed that the observed PL quenching is due to “coordination-induced aggregation caused PL quenching” mechanism.

Keywords: Coordination induced aggregation, L132 cells, Cytotoxicity, Stability of CDs, Photophysics of CDs, Static quenching, Green chemistry.

INTRODUCTION World nowadays produce the expanded polystyrene (EPS) massively in order to fulfil the needs and requirement of packaging industries. As the production of this product increase, the total amount of plastics that ends up in waste stream is in a similar trend. Polystyrene is one of the most widely used plastics, the scale of its production being several billion kilograms per year.1 Styrofoam appears to last forever, as it is resistant to photolysis and is non-biodegradable, require centuries perhaps millennia to decompose!2 As it is composed of more than 90% air,3,4 it is highly mobile and escapes from garbage bins and landfill, litter the streets or end up polluting water bodies. Tiny polystyrene globules could be orally consumed by animals leading to choking or intestinal obstruction. It could be fatal to the animals that swallow considerable quantities.5 Polystyrene foam is a major component of plastic debris in the ocean and are a source of hazardous additives for marine organisms.6 Marine animals higher up the food chain could eat the fishes that have consumed EPS, thus leading to bioaccumulation.7 It could be a potential health hazard for humans who are on top of the food chain considering that styrene, the monomer used in manufacturing EPS has been classified by the US National Institutes of Health (NIH) and the International Agency for Research on Cancer (IARC) as a possible human


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carcinogen.7 Many cities and counties (Washington DC, New York City, Los Angeles County, San Francisco, Taiwan, Portland, etc.,) have outlawed polystyrene foam.8 Therefore, optimizing the waste EPS by applying various practical approaches such as prevention, minimization, reuse or recovery is the urgent need. EPS foam is generally used once before disposal. Given that there is a high amount of energy embodied in foam products, their single use is particularly lavish. As it is bulky (90% air), EPS foam occupies a large storage space and increased cost of transportation despite produces merely a small quantity of polystyrene to recycle.7 This provides a low incentive for recyclers to consider EPS recycling. At present, it is more economical to produce new EPS foam products than to recycle it, therefore, the manufacturers would rather have the higher quality of fresh polystyrene over the recycled one.7 Hence, many curbside recycling agencies are not accepting the EPS goods. These valuable factors discourages recyclers from recycling the waste EPS. On the other hand, the incineration of these EPS materials is not economical as it requires a high temperature (up to 1000 °C) and a plenty of air (14 m3kg-1).9 Moreover, ninety diverse compounds were recognized in combustion effluents of EPS10 which is environmentally hazardous. The high flammability of EPS (classified according to DIN4102 as a "B3" product, meaning highly flammable or easily ignited) prevents the wide use of recycled EPS as thermal reduction material in building construction. Foamed polystyrene plastic materials have been accidentally ignited and caused huge fires and losses.11 Biodegradation and mineralization of EPS by plastic-eating mealworms;12

The

process

of

polystyrene

depolymerization13

(converting polystyrene back to its styrene monomer); conversion of waste styrofoam into engineered adsorbents for removal of cadmium, lead, and mercury in water14, are also gaining ground although, these are still in research or pilot stage. Although Huang et al.15 have claimed



that they have accidentally obtained the luminescent carbon dots (as by-product) from waste EPS during the synthesis of solid protonic acid, sufficient characterization data was not given (the TEM image is not clear enough to prove the existence of CDs). Moreover, the treatment of EPS involved tedious procedures including a multiple heating steps and treatment with concentrated mineral acids hazardous to the environment. Zhang et al.16 have reported the formation of CDs from the soot obtained from incomplete combustion of polystyrene foam. The major drawback in this method is that the CDs exhibits luminescence only in organic solvents such as THF, dimethyl benzene or toluene and it is non-luminescent in water. Hence, the obtained CDs can’t be utilized for biological applications such as bio-imaging, drug delivery, etc., and for fluorimetric sensing applications since the sensing of toxic ions will be meaningful only if the sensing achieved in aqueous medium. Therefore, the CDs produced by Zhang et al. can’t be employed for environmental aqueous samples which carry their CDs far away from the aspects of green chemistry. In this connection, we have synthesized highly luminescent, water-soluble carbon dots (CDs) from the waste EPS via a direct one step solvothermal synthesis. CDs are spherical nanoparticles with sizes below 10 nm, as a welcome member of the carbonaceous materials family, have attracted incredible attention since their accidental discovery,17 owing to their advantageous properties such as stable photoluminescence (PL), broad excitation spectra, multi-colored luminescence, surface tunable functionalities, low cytotoxicity, excellent biocompatibility, environmental viability, and water solubility. CDs are potential substitute for conventional toxic metal-based semiconductor quantum dots especially in biomedical applications.18-20 Several research groups have employed natural biomass to prepare fluorescent CDs via the hydrothermal carbonization technique.21-25 However, fluorescence quantum yields (QY) are not impressive


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( 9 can’t be purified by cellulose ester (CE) dialysis membranes as it will damage the membrane pores irreparably (pH tolerance = pH 2 to pH 9).35 So, one should go for other costlier membranes or should neutralize the pristine solution before dialysis which will introduce additional chemicals in the purification process. Hence, the optimization of EDA concentration saves the processing time and cost associated with the purification process. The CDs are freely soluble in polar solvents for example lower alcohols, water, dimethyl formamide, dimethyl sulphoxide, etc. while it is insoluble in non-polar solvents such as chloroform, dichloromethane, n-hexane, etc. The presence of heteroatomic surface functionalities may impart the polarity to the CDs surface leading to the selective solubility in polar solvents. Under optimized conditions, the obtained production yield of CDs is ∼225 mg per gram of EPS. As mentioned in the introduction section, EPS is consisting of more than 90 % air (usually pentane and steam) and less than 10 % polystyrene. According to this fact, of the 1 g starting material, polystyrene contributes only



∼100 mg by weight! The remaining mass of about 125 mg is due not only to the passivating agent (EDA) but also to the collective contributions from the solvent (CHCl3), and the trapped blowing agents (pentane and steam) used in the production of EPS (the presence of chloro, hydroxyl, and carbonyl groups in addition to amine functionality were confirmed by XPS and FTIR analyses). Hence, this synthetic method efficiently converts the potential pollutant, EPS into an eco-friendly material in a good yield, giving hope to produce the CDs commercially. The time period of thermolysis and the percentage of EDA are varied and optimized in order to produce CDs in a higher quantum yield and that are given in the Fig. S1a and S1b. Characterization We have carried out the TEM analysis in order to explore the morphological features of the CDs. The TEM image and the corresponding size distribution histogram were showed in the Fig. 1a and 1b respectively. From Fig. 1a, it is clear that the as prepared CDs possess spherical morphology and are monodispersed. Moreover, the CDs are well separated from each other. Gaussian fitting of the histogram in Fig. 1b reveals the statistical diameter of the CDs as 4.0 ± 1.2 nm on analyzing about 75 particles. The narrow FWHM, 1.4 of the Gaussian fit shows that the distribution is narrow. The XRD pattern of CDs is showed in Fig. 1c. A broad peak with 25° as the prominent 2θ value is observed which depicts the (002) lattice spacing corresponding to abounding sp3 defects of carbon based materials. The resultant d-spacing is higher (0.36 nm) compared with that of bulk graphite (0.33 nm) representing a weak crystallization. The blurred ring structure of SAED pattern of CDs obtained shown in the inset of Fig. 1a, further endorsing the amorphous nature of the CDs. Raman spectroscopy is a potential tool to estimate the crystallinity/amorphicity of a material and thus, the degree of disorder. The Raman spectrum (Fig. 1d) of the carbon dots exhibits two peaks at 1364 and 1559 cm-1, corresponding to the D


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and G bands respectively of carbon. The D band is accompanied with the vibrations of carbon atoms with pendent bonds in the termination plane of disordered graphite or glassy carbon. The G band corresponds to the E2g mode of the graphite and is associated to the vibration of sp2bonded carbon atoms in a two-dimensional (2D) hexagonal lattice. The ID/IG band intensity ratio is used for comparison of defects and disorder density in the CDs and also for quantification of sp3 hybridized carbon atoms. The calculated ID/IG ratio (the ratio between the integrated area of D peak and that of G peak) is 2.12. This indicates a high degree of defects formed by amination, chlorination, and oxygenation processes on the EPS during the solvothermal carbonization treatment. Although the values of ID/IG ratio is not clearly correlated with the concentration of nitrogen, chlorine, and oxygen, the obtained result shows a dominant influence of disorder on the ID/IG ratio. We have explored the diverse surface functionalities of the CDs with the aid of FTIR spectroscopy. The FTIR spectrum of CDs is shown in the Fig. 2. A broad peak from 3600 cm-1 to 3119 cm-1 is attributed to hydroxyl and amine functionalities. It is evident that the amino group from the passivating agent (EDA) is successfully transplanted to the CDs. The C-N stretching vibration at 1169 cm-1 further substantiates the presence of amino groups. The coexistence of hydroxyl absorption withal evince that the steam (blowing agent) and air trapped into the EPS beads were participated in the carbonization process. The inclusion of oxygen atom to the CDs is further supplemented by the presence of a strong absorption at 1630 cm-1 corresponding to carbonyl functionality and the presence of C–O stretching vibration at 1040 cm-1. The signal at 2955 cm-1 is attributed to the stretching vibrations of methyl/methylene groups at the surface of the CDs. The corresponding bending vibrations are observed at 1445 cm-1 (asymmetrical) and 1346 cm-1 (symmetrical). We presume the presence of well conjugated and/or aromatic C=C moieties as a result of a strong peak merged with the carbonyl peak at 1550 cm-1. The sp2



hybridized conjugated carbon core of the CDs may be the origin of this absorption peak. The CCl3 stretching vibration and CH2–Cl bending (wagging) vibrations are observed at 758 cm-1 and 1289 cm-1 respectively. The value 758 cm-1 is almost equal to the literature36 reported value of 759 cm-1 for CCl3 stretching. Therefore, the fragmentation scene of chloroform during carbonization can be reconstructed in such a way that the chloroform would produce CCl3, CCl2, and CCl fragments on carbonization and the fragments would be bound to the carbon core. The absorptions corresponding to CCl2 and CCl fragments may be merged to the broad absorption caused by carbonization domain centred at about 540 cm-1. However, the presence of CH2–Cl bending vibration at 1289 cm-1 reveals the existence of monochloro substitutions also. XPS can furnish indisputable information regarding the nature of bonds and chemical composition of a material. The XPS survey spectrum of our CDs (Fig. 3a) clearly show the presence of C (59.6%), O (12.8%), N (17.5%), and Cl (10.1%) peaks in the CDs. The C 1s peak was found to be at 284.8 eV, O 1s at 531.0 eV, N 1s at 399.6 eV, Cl 2p at 202.0 eV, and Cl 2s at 268.0 eV. A detail peakfitting analysis of the C 1s peak of the as prepared CDs is shown in Fig. 3b. High resolution C 1s spectrum was fitted to quantitatively differentiate the six different carbon bonding states: C– C/C=C (284.8 eV), C–N (285.8 eV), and C–O/C=O/C–Cl (286.5 eV). Fig. 3c is the high resolution O 1s spectrum of the CDs which on fitting, quantitatively establishes the three different bonding states of oxygen: C–O–C/C–O–H (531.2 eV), and C=O (532.7 eV). The presence of four different bonding states of nitrogen is confirmed by the quantitative fitting of high resolution N 1s spectrum (Fig. 3d). The possible bonding states of nitrogen are C– N/pyrrolic-N (399.3 eV), N-H (400.7 eV), and pyridinic-N (401.7 eV). The presence of pyrrolic and pyridinic nitrogens shows clearly that the passivating agent EDA is not only introduced amino groups on the surface of CDs but nitrogen atoms too to the carbon core which results in


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the formation of nitrogen doped CDs. In view of the remarkable quantum-confinement and edge effects, doping CDs with electron-rich N atoms could drastically alter their electronic characteristics and offer more active sites, thus producing new phenomena and unexpected properties. In the past few years, several methods have been developed for the advanced synthesis of N-doped CDs. The nitrogen content of our CDs is much greater than many previously reported34,37-43 CDs which are claimed as N-doped and/or amine functionalized. This fact further extents the materialistic applicability of our CDs. Presence of chlorine functionalities on the surface of CDs are confirmed by the appearance of Cl 2s and Cl 2p peaks at 201.0 eV and 268.0 eV respectively (Fig. S2). Our intension is to synthesis an amine functionalized CDs. However, surprisingly we ended up with amine, hydroxy and chlorine co-functionalized, nitrogen doped CDs. The Cl groups could change surface charge distribution of CDs, resulting in high potential at the surface. Therefore, it can facilitate an effective separation of photogenerated charges inside the CDs.44 Furthermore, the bandgap of CDs can be modulated by chlorine doping.45 Thus, chlorofunctionalized CDs are reported as very good candidates as photocatalysts44,46 and photovoltaic detectors45. However, literatures are very few on the synthesis of chloro-functionalized CDs.44-47 Reports are involving a separate chlorination step of CDs with thionyl chloride46 or hydrochloric acid45 or addition of chlorinating agents such

as phosphorous pentachloride44 together

with the carbon precursor. Even after these efforts, the resulting Cl-CDs are very poorly functionalized in terms of chlorine groups. The obtained atomic percentage of Cl is not more than 2 %. Hu et al.46 have reported that the Cl,P-CDs with ∼2 % of Cl and ∼4 % of P, behave as an excellent photocatalyst towards the degradation of methylene blue and methyl orange. Its efficiency is much better than commercial photocatalyst, TiO2 (P25). On the other hand, our CDs



consist of ∼10 % of Cl (highest chlorine percentage reported for CDs so far to our knowledge) and the synthetic method eliminates the need of external chlorination step and addition of chlorinating agent to the reaction mixture. In our case, the CHCl3 plays dual role; as solvent and as the chlorinating agent. It is noteworthy here that the CDs produced by carbonization of CHCl3 with EDA by reflux method47, consists only a trace Cl and N heteroatoms in it whereas in our case, the EPS serves as a carbon matrix and facilitates the functionalization. Consequently, the resulting CDs are rich in Cl and N. Therefore, the as prepared CDs can exhibit an excellent photocatalytic activity and the related experiments are underway at our laboratory. Optical Properties of the CDs Fig. 4a represents the UV-Vis absorption, excitation, and PL spectra of the CDs. The UV-Vis absorption extends up to 600 nm without noticeable structures. The aqueous solution of CDs exhibits an intense blue emission (inset of Fig. 4a) on the exposure of UV (365 nm) radiation. Two excitation maxima are observed in the excitation spectrum at 273 nm and 371 nm. From the previous studies,48 the peak at 273 nm is attributed to π−π* transition of aromatic C=C bonds in the core, while the peak at 371 nm attributes to n−π* transition of heteroatomic surface functionalities. Our CDs exhibit excitation dependent emission behaviour (Fig. 4b) like most luminescent carbon nanoparticles. The maximum emission is observed at 456 nm (λex = 380 nm). On normalizing the PL spectra of CDs recorded at different excitation wavelengths, we can clearly observe a bathochromic shift of PL maxima with respect to the excitation wavelength (Fig. S3). The fluorescence QY of the CDs is ~20 % (Fig. S4, Table S2 and S3) relative to that of quinine sulfate which is ~54 % in 0.1 N sulfuric acid. The emission wavelength dependent excitation spectra of CDs are shown in Fig. 4c which shows two distinct excitation peaks at 275 nm and 348 nm when the emission is monitored at 400 nm (black-lined spectrum in Fig. 4c). The


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former band is referred as C-band (core excitation) and the latter as S-band (surface excitation). The S-band maximum is shifted from 348 nm to 373 nm when the monitoring emission wavelength shifted from 400 nm to 500 nm (Fig. S5b). On the other hand, the C-band maximum didn’t shift at all as the function of monitoring emission wavelength (Fig. S5c). This observation infers that the excitation dependent emission behavior observed in our CDs is primarily due to diverse surface states rather than quantum confinement effect. The 2D fluorescence contour pattern (Fig. 4d) of the CDs exhibits two discrete contours (representing the PL due to C-band and S-band excitations) with emission centers appear at 463 nm (λex = 273 nm) and 455 nm (λex = 371 nm). There is no much difference between the position of emission centers i.e., the emission maxima are almost unchanged with respect to the excitation centre (whether C-band excitation or S-band excitation). In light of our previous works,26,48 this observation clearly evident that “even when the excitons are generated from the carbogenic core, they get trapped and relaxes at the surface defects before recombination”. The fluorescence lifetime of the CDs is obtained by the TCSPC method. The CDs are excited at 385 nm and the emission is monitored at 460 nm (Fig. 5). The decay profiles of the CDs at various emission wavelengths are best-fitted triexponentially (Fig. 5 and Fig. S6) and the corresponding parameters derived from the non-linear least square fitting are tabulated in the Table S4. The lifetimes of the CDs generated from the fitting are, τ1 = 0.34 ± 0.03 ns, τ2 = 2.08 ± 0.05 ns, and τ3 = 5.86 ± 0.05 ns. The mean lifetime (τmean) is calculated to be 4.11 ns. Our CDs can be a potential candidate for optronic and biological applications as evidenced by its observed nanosecond lifetime.49 Stability of the As-Prepared CDs CDs dissolved in demineralised water didn’t undergo any aggregation during the monitoring period of 90 days. The solution remains homogeneous and its fluorescence QY remains almost



the same (Fig. S7a) which supplements the rich surface functionalization of CDs. Hence, the CDs possess a good shelf-life in solution state. The CDs solution is irradiated under 365 nm UV radiation for 1 h to evaluate its tolerance towards photolysis. Fig. S7b shows that the CDs possess a stable luminescence during the monitoring period. This is an advantageous property for a material to be employed in bioimaging and drug delivery. The luminescence of the CDs is more or less immune to the change in ionic strength (0 to 2 M) of the medium (Fig. S7c). The slight variations are within the error limit as the error bars indicating. This is an important property for a material to tolerate various saline concentrations at in vivo conditions when examine its ability to serve as a probe in bioimaging, drug delivery, and nanomedicine. Apart from that this property makes our CDs as persistent fluorescence sensors in real aqueous samples. The pH impact on CDs PL is depicted in the Fig. S7d. The optimal luminescence is observed at pH 7 which is again a constructive property towards biomedical applications as most of our body fluids are exists in nearly neutral pH. Unlike most of the previously reported CDs whose luminescence is quenched at both acidic and basic extremes, the luminescence of our CDs is not affected to a large extent in acidic pH (from pH 5 to pH 1) which extends its materialistic applications where other CDs cannot be employed. However, the luminescence of our CDs drastically quenched at alkaline pH (from pH 9 to pH 13) like most of the formerly reported CDs. Elementary biocompatibility assessment by in vitro cytotoxicity studies The remarkable properties of our CDs like small size of about 4 nm (allows facile and rapid in vivo renal clearance hence cytotoxicity will be less50), photostability, enduring luminescence to a broad ionic strength, and a high luminescence QY at neutral pH compelled us to investigate its cytotoxicity to exploit these CDs for biomedical applications. As an elementary step, we have


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evaluated its cell viability on L132 cells (human normal lung cells). Our CDs exhibits an excellent cell viability (more than 95 %) on L132 cells even at customary dosages (

Green Synthesis of Multifunctionalized, Nitrogen-Doped, Highly

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