Synthesis and study of carbon quantum dots (CQDs) for enhancement of luminescence intensity of CQD@LaPO₄:Eu³⁺ nanocomposite

Highlights

• Synthesis of carbon quantum dots (CQDs) has been done using aloe vera biomass by facile carbonization method.

• The water soluble CQDs can be used for Plant imaging.

• The CQD@LaPO₄:Eu³⁺ nanocomposite was synthesized.

• The addition of CQDs enhances the photoluminescence intensity of composite remarkably.

Abstract

The carbon nanodots are the newly emerging areas of research which have applications in many fields such as sensors, catalysts, bioimaging etc. Their excellent photoluminescence emission in visible region has prompted many researchers to explore new synthesis routes and precursors for better quantum yield. In this work carbon quantum dots are synthesized by using aloe vera biomass by facile carbonization process. The aqueous solution of carbon quantum dots (CQDs) shows blue light emission upon excitation by 365 nm ultra violet radiation. The field effect scanning electron microscopy images showed CQDs as spherical quantum dots of the size around 10 nm. The transmission electron microscopy (TEM) bright field images confirmed the size of the CQDs. The CQDs thus prepared can be used for bioimaging of plant cells and as an ink for security printing applications. A CQD@LaPO₄:Eu³⁺ nanocomposite has been prepared and its photoluminescence property has been studied. The increased photoluminescence intensity of composite can be attributed to Förster Resonance Energy Transfer (FRET) from CQDs to Eu³⁺.

Introduction

The carbon nanodot can be considered as the inspiring new carbon nanomaterial and caused intensive research efforts in recent years like other forms of carbon such nanostructures as the fullerene, carbon nanotube, and graphene. These carbonaceous quantum dots, so-called CQDs, provide several advantages over traditional semiconductor-based quantum dots like luminescent emission, biocompatibility, less toxicity apart from that a time efficient, un-tedious, facile method of preparation. They can be produced inexpensively and on a large scale using many precursors by many approaches, ranging from simple soot produced by candle burning to sophisticated physical methods like laser ablation method. Due to their excellent luminescent properties, they are called as nano lights [1]. Their size is very small around 10 nm, even less than 10 nm. They were first obtained during purification of single walled carbon nanotubes in 2004 and later by using laser ablation method in 2006 [2,3]. The excellent photoluminescent properties of CQDs can be used in optical analysis, cellular imaging, biological sensing, in photonics, in ion detection [4], and in molecular tracing [5]. The fluorescent material should have high absorption coefficient, low toxicity and high quantum yield for bioimaging. The highly luminescent semiconductor quantum dots met the two requirements but fails in toxicity test as they contain heavy metals. These materials cannot be used as biological labelling materials. The best alternative is CQDs [6,7]. They exhibit tuneable fluorescence emission. This exclusive property of CQDs has been the natural driving force for extensive research expansion on this material. They can be surface passivated by several organic, inorganic, polymeric and biological materials. This process improves their physical and fluorescent properties. Their small size and biocompatibility enable them to be used as effective carrier for drug delivery; while their excellent catalytic and physiochemical properties make them feasible for many biomedical applications [8,9]. CQDs have a property of donating and accepting electron. Hence, they can act as antioxidants and pro-oxidants. This helps in scavenging the free radicals during metabolism which are the result of interaction of biomolecules with molecular oxygen [10]. The other carbon nanomaterials such as graphene are already studied for the elimination of free radicals which are the major cause of diseases viz. Cancer, Alzheimer and Parkinson's diseases [11,12].

Since the discovery of CQDs during the purification of single walled carbon nanotubes by Xu et al., in 2004 [2], many advantageous properties led the researchers all over the world to prepare carbon nanodots and study their excellent photoluminescence properties. They were found suitable for tremendous applications right from sensors, catalysis, and fluorescent probe to bioimaging and targeted drug delivery in everyday life. Moreover, simple precursors were tried and innovative methods were invented to better the mechanical and optical properties. Plants, fruits and food products such as papaya juice, banana juice, apple juice, cabbage, unripe peach, waste frying oil, coffee beans, egg, flour, honey, soya milk, sugarcane bagasse pulp etc. [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]] were used to prepare carbon quantum dots. Till now, many methods of synthesis have been tried and tested. Carbon dots have been reported to be synthesized by Sun et al. using laser ablation. The research group prepared a carbon target through high temperature (900 °C) treatment of graphite powder/cement mixture and then a laser ablation was applied. The as prepared carbon dots didn't initially showed fluorescence until passivated by hydrocarbon chains [3]. After this, first report of synthesis of carbon dots, many precursors which have carbon containing substrates were used to prepare carbon dots. The precursors used were amino acids, proteins, organic solvents like toluene [29], various hydrocarbon containing saccharides etc. Yu et al. produced graphene sheaths from toluene, which subsequently produced fluorescent carbon dots. Y P Sun et al. studied doping of carbon dots with semiconductor salts such as ZnS and ZnO before the surface passivation and showed that the luminescence yield has been increased [30]. S T Lee and co-workers used graphene electrodes and used electrochemical method to synthesize carbon dots. Various colours observed were ascribed to variations in particle size and surface defects [31]. Q Huo et al. produced carbon dots from natural source of carbon such as wood and after surface passivation using different amine terminated organic agents produced a water soluble carbon dots [32]. Microwave method was adopted by Y Zhang and co-workers to prepare carbon dots using calcium citrate and urea. Their sample showed luminescence in both solid and liquid phase which is somewhat unique result in this field as the carbon dots luminescence is quenched in solid medium [33]. Hydrothermal has been become the most common method for preparation of carbon dots. This process facilitates condensation of the carbonaceous building blocks and crystallization of the graphitic core [27,34,35]. The chemical vapour deposition (CVD) method was used to produce carbon quantum dots using C₂H₂ as carbon source. A mixture of C₂H₂ and Argon was introduced in a quartz tube at 1000 °C and the deposited product was dissolved in N, N-dimethylformamide and was filtered to remove impurities [36]. Mitra et al. used solvothermal method for preparation of carbon dots [37]. Rahy and co-workers used combustion method for preparation of carbon dots [38]. Wu et al. produced functionalized carbon dots using petroleum coke [39]. Y Wang et al. produced nitrogen-sulphur co-doped carbon quantum dots by microwave assisted synthesis method and discussed its application for bioimaging [40]. For bioimaging and drug carrying the use of carbon dots was suggested by numerous research groups all over the world [[41], [42], [43], [44]]. The fact that carbon dots can be surface passivated by organic and inorganic ligands opened up new area of research and many research groups investigated this possibility. Yu et al. produced high fluorescent carbon dots doped with gadolinium for fluorescent and magnetic bioimaging probe [45]. Carbon dots grafted with amine which can have tailored surface and high photoluminescent yield was prepared by M. Li et al. [46]. Polyamine functionalized carbon dots were prepared for sensing application by Y Dong et al. [47]. Carbon dots are thus advantageous in terms of their water solubility, physiochemical and photochemical stability, excellent biocompatibility and high optical emission property. The increasing use of these carbon dots are opening new possibilities in terms of new precursors and new synthesis methods. A new approach can be explored which emphasize the optical emission in near IR spectral regions. This can be effective in tissue penetration.

The quantum dots were studied in last two decades due to their tuneable and narrow emission spectrum and bright luminescence. But they are not used for plant imaging. The main reason being the toxicity of quantum dots which contains heavy metals. This fact reduces the application area of quantum dots. Therefore, the necessity of development of new fluorescent material arises as alternative to semiconductor quantum dots. Use of natural products may lower the toxicity of the synthesized material for biomedical applications. In this work, the synthesis of carbon quantum dots is done using natural precursor aloe vera biomass and a facile carbonization method was used. The literature reveals that there are very few reports on enhancement of PL of phosphor when it is in the form of composite prepared by mixing carbon dot with phosphor powder [[48], [49], [50], [51]].

Lanthanum orthophosphate (LaPO₄), which is a monazite, is used as a high-quality phosphor in tricolor luminescent lamps. It can be doped with rare earth ions for different colour emission. Its myriad applications include lasers, sensors, tricolor lamps and White Light Emitting Diodes (WLEDs) and catalysts to name a few. It has many properties, such as high thermal stability and high index of refraction etc [52,53].

In this study, we prepared LaPO₄:Eu³⁺ phosphor and studied the effect of CQD on the photoluminescence emission intensity by adding CQD to it. Such facile approach can be very useful in the preparation of optical devices phosphors.