Chemiluminescence of graphene quantum dots and its application to the determination of uric acid
Introduction
Graphene, a single layer of carbon atoms in a honeycomb structure, have gained considerable interest because of their fascinating physical and chemical properties [1], [2]. A new member of graphene family, which has been discovered very recently, is graphene quantum dots (GQDs). They are graphene sheets smaller than 100 nm that exhibit numerous unique physicochemical properties due to the pronounced quantum confinement and edge effects. For example, unlike graphene that does not exhibit photoluminescence (PL) because of its zero-bandgap, GQDs prepared via different approaches can emit intense PL with different colors. Compared to the common carbon-based photoluminescent nanomaterials (carbon dots), GQDs have some excellent characteristics, such as higher surface areas, larger diameters and better surface grafting properties. Also, unlike carbon dots which usually contain amorphous carbon, GQDs clearly possess graphene lattices inside the dots regardless of the dot size [3]. Moreover, compared to semiconductor quantum dots, GQDs show considerably lower toxicity, excellent solubility, chemical inertia, stable PL, high electrical and thermal conductivity [3], [4], [5]. As a result of these superior properties, GQDs have attracted the growing attention in many areas and applications such as bioimaging [6], [7], photovoltaic devices [8], [9], electrochemical catalysis and biosensors [10]. However, there are only a few reports on the fluorescence [11], [12], [13], [14] and electrochemiluminescence studies of GQDs [15], [16]. To the best of our knowledge, the chemiluminescence (CL) properties of GQDs have not yet been studied, though the CL of carbon dots has been reported before [17], [18], [19], [20], [21], [22].
CL, light emission induced by chemical reactions, is an attractive analytical method due to several advantages such as high sensitivity, simplicity of instrumentation, low detection limit, large calibration range and short analysis time. Recently, the CL studies have been extended from traditional molecular systems to the nanoparticle-based systems. In these systems, nanoparticles can participate as a catalyst, reductant, luminophore, and energy acceptor [23], [24], [25], [26]. Among nanoparticles, some QDs such as CdSe/CdS [27], CdTe [28], [29], [30], [31] and CdS [32] have been applied as the CL emitters. However, the inherent toxicity of Cd-based semiconductors may raise serious health and environmental problems [33], [34]. Therefore, it is necessary to find a safer alternative to semiconductor QDs with good CL activities.
Uric acid is the end product of purine catabolism in humans. At physiological pH, uric acid is mostly ionized and therefore is present mainly as the urate ion. When serum urate level exceeds the solubility limit, sodium urate crystallizes in soft tissues and joints and causes an inflammatory reaction, gouty arthritis. Other disorders of purine catabolism include Lesch–Nyhan syndrome, von Gierke׳s disease, and hypouricemias [35]. Therefore, monitoring of uric acid in human plasma and urine is of great importance in the physiological investigations and disease diagnosis.
In the present work, direct CL of GQDs induced by some common oxidants was investigated. The mechanism of GQD CL system was proposed based on the fluorescence and CL emission spectra. Moreover, based on the diminishing effect of uric acid on the GQDs-Ce(IV) CL system, a simple and sensitive method was proposed for the determination of trace amount of uric acid in human plasma and urine samples.
Section snippets
Apparatus
The CL signals were monitored by LUMAT LB 9507 chemiluminometer (Berthold; www.berthold.com). CL spectrum was recorded by RF-540 spectrofluorimeter (Shimadzu, Japan) using flow mode with the excitation light source being turned off. The fluorescence spectra were also recorded by the same instrument under normal conditions. UV–vis spectra were recorded on a Cary-100 Spectrophotometer (Varian; www.varianinc.com). The size and shape of GQDs were characterized by transmission electron microscopy
Characterization of GQDs
In this work, a simple carbonization method was used for synthesis of GQDs. The TEM image of synthesized GQDs (Fig. 1a) shows that they are of spherical shape and nearly mono disperse nanoparticles with size distribution in the range of 5±2 nm. The XRD pattern of GQDs shows a broad (0 0 2) peak around 25θ (0.36 nm), suggesting that carbonizing citric acid produces graphite structures (Fig. 1b). The FT-IR spectrum of GQDs is shown in Fig. 1c. The peaks of COO− at about 1574 and 1397 cm−1 and
Conclusion
In this work, we demonstrated that Ce(IV) as a common CL reagent could directly oxidize GQDs to give rise to a relatively strong CL emission. A mechanism for the CL system was proposed, which involves the reaction of Ce(IV) with GQDs to inject holes into them. Then, the thermally excited electrons annihilate with the Ce(IV)-injected holes to produce the excited-state GQD⁎, which acts as the final emitter in the CL system. A new analytical method for the determination of uric acid was developed
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