| As a common luminescence phenomenon, chemiluminescence (CL) is based on a chemical reaction, and has been an attractive topic of intensive researches in various fields because of its high sensitivity, wide linear dynamic range, simple instrumentation, fast analysis, and low background. Among many well-established CL reagents, luminol is the most frequently used due to the relatively high luminescent quantum yield and good water solubility. Recently, with the development of nanotechnology, the CL of luminol has been extended to nanoparticles (NPs) systems in addition to traditional molecular or ionic systems. Higher sensitivity and stability can be achieved by using the NPs owing to their large surface area and special structures. However, the NPs-participated CL of luminol is still limited. On the other hand, many reported literatures on luminol CL only contained the assumption on the mechanism or proposed the possible luminescent processes on the basis of a few experimental data. The reaction mechanism was rarely studied by experimental methods, so we can hardly get the detailed information of the reaction intermediates. Therefore, it is an important issue to develop a new NPs-involved CL and further investigate its chemiluminescent mechanism. In this contribution, we attempt to establish the carbon-and metal chalcogenide nanomaterials-catalyzed luminol CL systems and carry out a series of studies on their enhancement mechanism. The main contains are listed as follows:1. Graphene oxide (GO) was found to enhance the CL of luminol-H2O2system mainly through the intermediate of1O2, which was greatly different from the traditional catalyst in such CL system that occurred in a strongly basic medium through the intermediates such as superoxide anion radical (O2-) and hydroxyl radical (OH-). In this work, GO could catalyze the luminol-H2O2CL in a weakly alkaline medium, and the GO-enhanced CL is markedly stronger than that of the other carbon materials including CNPs, MWCNTs, or SWCNTs, which is attributed to the high specific surface area of GO (calculated value,2630m2/g). Moreover, the effect of GO on luminol CL seems to be much stronger than that of RGO, owing to the oxygen-related groups of GO which are indeed a reactive part and crucial for luminol CL. Furthermore, taking GO-catalyzed luminol-H2O2CL as an example, we used CL spectral, UV-visible absorption spectral, and EPR spectral measurements and the effects of various free radical scavengers on the GO-enhanced luminol CL to identify the possible CL enhancement mechanism. The results showed that the enhancement took place mainly through the intermediate of1O2. It is well known that GO has a large conjugated structure, which can accelerate electron-transfer processes and the generation of radical species, as well as improve their stability. That is, the addition of GO into luminol system could not only catalyze the decomposition of H2O2to produce OH· and O2·-, but also facilitate the recombination reaction between them to form a high yield of1O2on the surface of GO. Then, the resulted1O2then reacted with luminol, producing an endoperoxide which decomposed to the excited state3-aminophthalate anions (3-APA*), and giving rise to light emission with the maximum wavelength at440nm. So the CL reaction between1O2and luminol to produce the3-APA*must be dominant in the present CL syetem. This work enriches greatly luminol CL mechanism, which would be of great potential applications in nanomaterial-based analytical chemistry.2. Cetyltrimethyl ammonium bromide passivated carbon nanodots (CTAB-CDs) can be used as excellent catalysts to dramatically enhance the CL intensity of luminol-H2O2system in NaOH medium. More importantly, this CL enhancement takes place mainly through the intermediate of1O2, which follows a different mechanism from traditional reports. CDs attract growing attention in many fields because of their excellent catalytic activity, which may result from the interactions between their unique surface structure and the reagents. In this work, we prepared a new kind of CTAB-CDs through the hydrothermal method, using fullerene (C60) as the raw material in the presence of CTAB. The CTAB-CDs can enhance the CL intensity of luminol-H2O2system in NaOH medium up to20times, and show a better CL catalytic ability than that of GO reported in our previous work. The CL spectra, UV-vis spectra, EPR spectra, and transmission electron microscopy (TEM) images before and after the CL reaction, as well as the effects of various free radical scavengers on the CL intensity were all conducted to identify the possible CL enhancement mechanism. Similar to GO, the enhanced CL by CTAB-CDs was still attributed to the1O2-induced luminescent mechanism. During the process of hydrothermal synthesis of CTAB-CDs, the conjugate structure of Cf,o has not been completely destroyed, and thus the as-formed CTAB-CDs may keep partly the conjugate structure of C60. The existence of the conjugated structure of CTAB-CDs could decompose H2O2effectively into OH-and O2-, which then participated in the radical recombination reaction to form plenty of O2on the surfaces of CTAB-CDs. Therefore, the enhanced CL of the CDs-based luminol system in NaOH solution was primarily caused by1O2, rather than other reported oxygen radicals. The present study is beneficial not only for gaining a better understanding of the1O2-induced luminol CL mechanism in a strongly alkaline medium, but also displaying the unique surface property and chemical reactivity of CDs.3. The semiconductor chalcogenide nanomaterial, copper selenide (Cu2-xSe), was found to catalyze the chemiluminescent reaction between luminol and H2O2, and the key intermediates induced luminescence enhancement were the OH-and O2-. The experimental results showed that Cu2-xSe NPs could enhance the CL signal of luminol-H2O2system more than500folds in alkaline medium. The strong catalytic ability of this kind of NPs is probably attribulted to the critical roles of copper element or copper defects. Based on the CL spectra, UV-vis spectra, EPR spectra, and scanning electron microscope (SEM) images before and after the CL reaction, as well as the effects of various free radical scavengers on the CL intensity, we considered that OH-and O2-played an important role in luminol CL enhancement. In other words, these two kinds of free radicals did not further react with each other to form O2on the surface of Cu2-xSe NPs. Therefore, OH-and O2-, instead of1O2, were the key species for the emission processes, which was identical with the common nanocatalytical systems.4. The structure of NPs, especially the large conjugate structure, has a great influence on the1O2-induced luminescence mechanism. We also studied the catalytic activity of AuNPs, AgNPs and GO/Cu2-xSe in strong alkaline conditions. The results indicated that1O2did not generate on the surfaces of both AuNPs and AgNPs. On the contrary, abundant1O2was observed on the surface of GO/Cu2-xSe. We supposed that the NPs used as catalysts could accelerate the electron transfer and decomposition of H2O2to produce oxygen-related intermediates, and may play an important role in making them react with each other to form1O2as well. As for the NPs including GO, CTAB-CDs, and GO/Cu2-xSe, their catalytical processes can be the typical1O2-induced luminescence mechanism because of the presence of the conjugated structure. However, for the Cu2-xSe, AuNPs, and AgNPs, the generation of1O2from OH·and O2-could not occur due to the absence of the large conjugated structure, and the role of these NPs was only to accelerate the electron transfer and the decomposion of H2O2. Therefore, the large conjugated structure on the surface of NPs is a critical factor, which directly determines the key radicals in the process of luminol CL.The above experimental results indicate that, the NPs are introduced into the luminol CL system as the catalysts, which can remarkably expand the theory and applications of CL. As for the mechanism, the existence of large conjugated structure of the NPs can accelerate the generation of free radicals, and further make them recombination. That is, the NPs with the large conjugated structure can catalyze the electron transfer and the decomposition of H2O2to form OH·and O2-,and then they react with each other to form abundant1O2. In summary, the research results described above would be important for deeply understanding the CL mechanism of luminol and the properties of NPs, and helpful to expand their applications in a variety of fields. |
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