Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Solvatochromism in highly luminescent environmental friendly carbon quantum dots for sensing applications: Conversion of bio-waste into bio-asset
Graphical Abstract
Introduction
Since the first report of discovery of fluorescent carbon nanoparticles in 2004, the search for benign carbon based nanomaterials having optical properties similar to those of semiconductor quantum dots are still continuing [1]. And recently significant research interests have been seen on carbon quantum dots (CQDs) due to their fascinating optical properties which come from π-conjugated (sp2 hybridized) graphitic backbone in the presence of sp3 hybridized functional groups [2], [3], [4], [5], [6], [7], [8], [9], [10]. Luminescent CQDs, in compared to the conventional luminescent organic dyes and semiconductor quantum dots, exhibit some advantageous properties, such as easy and low-cost synthesis procedures, availability of easy methods of surface modification, high photostability, high aqueous solubility and inherent biocompatibility [2], [3], [7]. And these exceptional characteristics of CQDs may be utilized advantageously in designing optode for biophotonic and bio-sensing applications [4], [5]. So far, a variety of techniques such as, arc discharge [1], microwave irradiation [6], electrochemical oxidation [8] and oxygen plasma treatment [9] have been developed for synthesis of such highly luminescent CQDs.
However, the previous synthesis procedures as mentioned above are not the ideal choice for greener production of luminescent CQDs, as most of these techniques rely either on the use of strong chemical reagents [5] or require complicated reaction constraints/ mechanisms or both. In most of the earlier mentioned methods [1], [2], [3], [4], [6], [7], [8], [9] additional expensive surface passivation agents are required in order to enhance the fluorescent quantum yield of the synthesized luminescent nano carbons to achieve the goals of the application. Recently, Liu et al. [5] have reported sensing application of carbon dots synthesized by chromatographic fractions technique. Whereas, Ali et al., have reported synthesis of hydrophobic fluorescent carbon nanoparticles (CNPs) from ascorbic acid and oleylamine solution [10] and then CNPs are turned into hydrophilic one by functionalization with PEG, primary amine, glucose, etc. However, a drastic difference in PL quantum yield (PLQY) between hydrophilic and hydrophobic CNPs is reported [10]. However, green and facile approaches for the synthesis of highly luminescent biocompatible CQDs with interesting optical properties along-with their fluorescent based sensing applications is rarely reported. Therefore, hydrothermal method for synthesis of CQDs has gained much-anticipated attention among the scientific community due to its cheap and green approach compared to other synthesis techniques [2], [3]. In addition to that, luminescent carbon nanostructures can also be synthesized successfully from different bio-precursors by using the hydrothermal technique and one can avoid the requirements of complicated post-synthesis treatments.
Recently, there are some reports on green syntheses of luminescent carbon nanostructures for different biophotonic sensing applications [11], [12]. The natural occurrence of fluorescent CQDs in honey has been reported very recently by Mandani et al. [13]. And such study has propelled researchers to design and manufacture of more sustainable and eco-friendly soft carbon nanostructures for more reliable applications in bio-photonics. However, further study on designing and synthesis of functional nanomaterials for highly reliable sensing of organic compounds and toxic ionic environment has a major space in terms of technological aspects. The exhaustive production and study on luminescent sensing probes with innovative detection parameters are more desirable [14], [15], [16], [17] than the conventional analytical techniques, such as Karl-Fisher titration [18], gas chromatography [19] in terms of selectivity, low background noise, easy fabrication procedure and cost-effective instrumental requirements [20], [21], [22]. However, due to the possibility of tailored designing of fluorescent nanostructures, highly sensitive and selective detection of analytes by using nano fluorescent probe has become a most effective technique now-a-days. Most importantly, luminescent carbon nanostructures in photo-excited state, is an reliable candidate for the purpose of fluorescence based sensing applications as it can act as a good electron donor or accepter [3], especially in the case where electron acceptor or donating ions, solvents and gases are the detection targets.
The time resolved photoluminescence (TRPL) experiment have been utilized earlier by some researchers to find the effects of energy redistribution, mechanisms of relaxation among the emitting states, and the origin of spectral migration of the fluorescence spectra of CQDs [23], [24], [25], [26]. TRPL measurement results can also be utilized while using CQDs in sensing applications. Recently, Kalytchuk et al. have reported monotonic shortening of the apparent PL lifetime of carbon dots with increasing temperature and they have utilized their temperature-dependent PL lifetime data to demonstrate the application of carbon dots in intracellular nano-thermometry [26]. Another interesting phenomenon, the ability of fluorophore to change their absorption and/or emission spectra as a result of a change in solvent polarity, known as “solvatochromism”, can be utilized industriously for the purpose of sensing applications [27], [28]. More importantly, the interaction between the polar functional groups attached to the surface of a fluorophore having a π-conjugated backbone as well as those present in the dispersed medium may play key roles in achieving the solvatochromic effect. The impact of π-conjugated structures grafted with electron donor and acceptor functional groups on solvent polarity dependent fluorescence property has been reported earlier [29], [30], [31]. Recently, solvatochromic effect of a new π-conjugated dye based on phenylene(poly)ethylene for sensitive detection of low level of water in organic solvents has been demonstrated by Guan et al. [31]. Although, during the last a few decades, solvatochromic behaviors of some typical dyes and their derivatives have been studied enormously, only a few of them have been applied in practice due to a number of limitations in terms of sensitivity, photostability, chemical stability and toxicity [31], [32], [33].
Observation of solvatochromic effect in dispersible and highly luminescent semiconductor quantum dots of CdSe has been reported by Leatherdale et al. [34]. However, in-vivo application of CdSe as a fluorescent sensing probe is limited due to their toxic nature and further modification in structural and chemical parameters are essentially needed [35]. Solvatochromism in single-walled carbon nanotubes has also been reported by Choi et al. [36]. However, carbon nanotubes still have several drawbacks, such as their insolubility in most organic solvents [37] and required functionalization for achieving the hydrophilicity which limits their hands-on application as a sensing probe [38]. Thus new approach for synthesis of highly luminescent, water dispersible and bio-compatible novel materials having solvatochromic PL emission property for fluorescence based polarity and ionic sensing applications is urgently needed. Solvatochromic PL (SPL) emission in fluorescent carbon nanostructures including in CQDs is a rarely reported phenomenon and only a handful of reports are available, so far [29], [39], [40], [41]. However, recently, Sciortino et al. [40] have used solvatochromism as an important tool to find the origin of excitation dependent PL emissive behavior of heavily N-doped CQDs with carbon nitride core. Therefore, a fresh investigation on SPL and optical properties of CQDs, synthesized by using bio-based precursor material will be very interesting, in view of its fundamental aspects as well as for demonstrating fluorescence based sensing applications.
In the present study, we have used an eco-friendly approach i.e., hydrothermal technique for synthesis of water soluble, self-passivated luminescent CQDs from naturally available bio-waste, namely Aegle marmelos (popularly known in India and in some other countries as “Bael”) leaves powder and a detailed study on SPL emission property of CQDs has been carried out for sensing applications. The luminescent CQDs having 3–8 nm sizes have exhibited excitation dependent PL emission behavior with PLQY of 3% in an aqueous medium. Interestingly, an enhancement in PLQY up to 22% has been achieved by dispersing the CQDs in tetrahydrofuran (THF) with permittivity (ε) = 7.5. Also the PLQY of the synthesized CQDs has been found to follow an exponential decaying nature with increase in ε of different polar dispersants. It has been found that the excitation wavelength (λex) dependent PL emission property of the CQDs in an aqueous medium is strongly related to the solvent polarity, when λex is > 325 nm. Whereas, PL emission behavior of the sample for λex < 325 nm has been found to be dominated by the concentration dependent rigidity aggregated crosslinking enhanced emission (RACEE). The observed RACEE is occurring due to the presence of functional groups (act as sub-fluorophore) within the π-conjugated graphitic core of CQDs. To find the applicability of the CQDs as a highly precise fluorescent sensing probe, the photo-physical stability of the synthesized sample has also been tested by varying different experimental conditions.
A flexible PVA-CQD film has also been developed to demonstrate the solvent sensitive emission characteristics of the synthesized sample in solid state form, under irradiation of 365 nm UV light. By using an indigenously developed experimental setup it has been shown that the PVA-CQD solid and flexible luminescence system emits intensity modulated blue light (~ 450 nm) when it is immersed in bulk organic solvents of diverse polarities. This observation supports the fact that PVA-CQD flexible film can be used as a solid state photonic sensor for detection of organic liquids in future. On the other hand, the SPL emission property of the synthesized bare CQDs sample, at an excitation wavelength of 325 nm have been demonstrated after dispersing it in different non-polar (such as cyclohexene, CH), polar protic (such as, MeOH, EtOH and DI water) and polar aprotic solvents (such as THF, dimethylsulfoxide, acetone and DMF,). The SPL emissions of CQDs in polar aprotic solvents have been found to occur due to non-specific dipole-dipole interaction in accordance with the Lippert-Mataga model. Whereas, when CQDs are dispersed in polar protic solvent, hydrogen bonding (HB) along-with dipole-dipole interactions have been found to play important roles.
Here, we have demonstrated the detection of organic solvents namely, EtOH and THF from their aqueous binary mixture by using the SPL emission behavior of the synthesized material and achieved the sensitivity of 1% and 0.9% molar fraction for EtOH and THF, respectively. Furthermore, the effects of the presence of different metal ions (Zn2 +, Ag+, Al3 +, Cd2 +, Mn2 +, Ni2 +, Cu2 + and Fe3 +) on PL emission characteristics of the aqueous dispersion of CQDs have been studied in the present work. PL emission intensity of the CQDs loaded with Fe3 + ions has been found to be quenched to the maximum value of 30% as compared to that of the unloaded CQDs. Whereas the PL emission intensity of the CQD loaded with other metal ions remains almost unaltered. This observation suggests the selectivity of the synthesized carbon nanostructures towards Fe3 + ions. We have also demonstrated possible application of CQDs in Fe3 + ion sensing even in real water samples with a limit of detection (LOD) of 0.12 μM. Thus in the present study we have demonstrated the conversion of a bio-waste to a bio-asset via green synthesis approach and synthesized highly luminescent CQDs. The detailed investigation on SPL emission property of the samples has added a new dimension to the low-cost synthesis of CQDs and its fluorescent based sensing applications and it could be an ideal replacement of traditional dye molecules and semiconductor nanostructures in future. On the other hand, demonstration of solvent sensitive emissive behavior of flexible PVA-CQD films can provide a new concept of polarity sensing by using CQDs in solid state form.
Section snippets
Materials
The fresh “Bael” green leaves used as precursor for the synthesis of CQDs has been collected from National Institute of Technology (NIT) Durgapur campus (India). Silver nitrate (AgNo3), Iron chloride (FeCl3.6H2O), Cadmium nitrate (Cd(NO3)2.4H2O), Aluminium nitrate (Al(NO3)3.9H2O), Zinc nitrate (Zn(NO3)2.6H2O), Manganese chloride (MnCl2.2H2O), Copper sulphate (CuSO4.5H2O), Nickel Chloride (NiCl2.6H2O), Methanol (MeOH), N,N-Dimethylformamide (DMF), Tetrahydrofuran (THF), Acetone (Ace) and
Structural Characterization of CQDs
Aegle marmelos (bael) is a natural herb and its leaves are commonly used around the world as a natural medicinal product. In the present study, Water soluble, self-passivated luminescent and solvatochromic CQDs have been synthesized by using simple hydrothermal process by using Aegle marmelos leaves as a starting material. The typical synthesis procedure has been schematically illustrated in Fig. 1. The initial change in colour of precursor solution from dark yellow to dark brown after
Conclusions
In summary, here we have reported a simple and facile hydrothermal technique for synthesis of a carbon based fluorescent nanostructure, namely CQDs from Aegle marmelos leave, a naturally available green precursor material. The CQDs thus synthesized exhibit excitation dependent PL emission property with the highest PLQY of 22% and a considerable enhancement in PLQY by a factor of 7 is demonstrated when the synthesized sample has been dispersed in THF as compared to that in DI water. The PLQY
Acknowledgements
Authors are thankful to CSIR, Govt. of India for the financial grant no. 03(1328)/14/EMR-II, dt. 03.11.2014. AP is also thankful to CSIR for the maintenance scholarship. Authors are thankful to Central Research Facility (CRF), IIT Kharagpur for TEM, Raman spectroscopic characterizations and the DST-FIST funded XPS facility at Department of Physics and Meteorology, IIT Kharagpur for XPS characterization. Authors are thankful to the reviewers for their fruitful suggestions. AP is also thankful to
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