Molecularly Imprinted Polymers On Quantum Dots For Multichannel Rapid Determination Of Neurotransmitters

Norepinephrine (NE) and epinephrine (E) are two major monoamine neurotransmitters with complementary actions in human bodies. Simultaneous detection of the changes of NE and E levels in biological fluids is clinically important for diagnosing pheochromocytoma, paraganglioma, neuroblastoma, and for evaluating hemodynamic function in patients with intensive care. The reported methods, such as HPLC-MS, UPLC-MS/MS and capillary electrochromatography, need expensive instruments, professional and laborious operations.Quantum dots (QDs) have been widely employed as effective fluorescence probes in the analysis of metal ions, small molecules, and even biomacromolecules. However, researchers have found that the selectivity of QDs sensors is poor, which limits their usage in detecting real samples. Molecular imprinting is a promising strategy to design a matrix that can be tailor-made materials with high selectivity for a target molecule. The resulting molecularly imprinted polymers (MIPs) show a higher affinity for the template molecule than for other structurally related compounds. Lately, composite materials with characteristics of high selectivity and sensitivity were constructed by anchoring MIPs on the surface of QDs (QDs@MIPs), and have attracted much attention in the fields of bioanalysis, pharmaceutical and environmental analysis.In this research paper, a unique system to simultaneously detect NE and E by QDs@MIPs was developed. The main parts of the results are summarized briefly as follows: 1. Preparation and application of NE-QDs@MIPs. NE-QDs@MIPs using NE as template were synthesized on the surface of CdTe/SiO2 QDs by sol-gel method. The as-prepared NE-QDs@MIPs were characterized by Fourier transform infrared spectroscopy, transmission electron microscopy and fluorescence spectroscopy. The NE-QDs@MIPs probe had distinguished selectivity and high binding affinity to the template molecule. Under the optimum conditions, the fluorescence quenching degree of NE-QDs@MIPs showed good linearity with correlation coefficient of 0.9950 in the concentration range of 0.04-10μM of NE. The limit of detection was 8 nM (3σ/K). The proposed method was applied to the analysis of NE in rat plasma. The spiked recoveries of NE were in the range of 98.20-106.1% with relative standard deviation ≤9.8%.2. Synthesis and charateriazaion of CdTe/CdS/ZnS/SiO2 QDs. In order to determine NE and E simultaneously by QDs@MIPs, NE-QDs@MIPs and E-QDs@MIPs should be excited at the same excitation wavelength, and their fluorescence signals should be monitored at the same time without mutual interference. Since polymeric layer formed on the surface of QDs is not luminous and the optical properties of QDs@MIPs depend on the original QDs, the QDs used in simultaneous detection should own the following characteristics:(1) excitation spectra with broad overlap; (2) fluorescence spectra at the maximum emission wavelength without interference. According to the quantum size effect, the luminescent properties of QDs can be tuned by properly choosing the material and size of them. Growing a shell or double shells of semiconductor materials with wide band gap on the core QDs is an effective way to increase the size of QDs. CdTe/CdS/ZnS QDs was selected for the study, and it was coated with silica by one-pot method before the polymerization reaction. CdTe/CdS/ZnS/SiO2 QDs with the maximum emission wavelength at 621 nm was prepared. The maximum emission wavelength difference between the as-prepared CdTe/CdS/ZnS/SiO2 QDs and CdTe/SiO2 QDs used in the preparation of NE-QDs@MIPs was 94 nm, and the fluorescence spectra at the maximum emission wavelength had no interference.3. Simultaneous detection of NE and E. When excited at the same wavelength (365 nm), NE-QDs@MIPs and E-QDs@MIPs emitted green and scarlet light, respectively. The mutual interference of fluorescence intensity of the two kinds of QDs@MIPs at the maximum emission wavelength was very small. NE-QDs@MIPs and E-QDs@MIPs adsorbed the corresponding template molecules much higher than the structural analogs. It is noted that although the structure of NE and E differs by only a methyl group, the corresponding QDs@MIPs had good selectivity ability toward the template molecule. The molecular size of NE was smaller than that of E and it was easier for NE to get into the cavities of E-QDs@MIP, but the specific recognition sites of E-QDs@MIP were not complementary to NE. As for E, the steric hindrance caused by the methyl group not only reduced its chance to enter the recognition cavities of NE-QDs@MIP, but also affected the formation of the hydrogen bonding between E and the functional group on the binding sites of NE-QDs@MIP. When only NE was present in the mixture of the dual-color QDs@MIPs, the fluorescence intensity of NE-QDs@MIPs decreased with the increase of NE while the fluorescence intensity of E-QDs@MIPs almost remained unchanged. Similarly, when E alone existed in the analysis sample, only the fluorescence intensity of E-QDs@MIPs decreased with the increase of E and the fluorescence intensity of NE-QDs@MIPs almost remained unchanged. Thus, the QDs@MIPs with two different colors could be used to detect the two monoamine neurotransmitters simultaneously. Under the optimal conditions, the fluorescence intensity of each kind of QDs@MIPs decreased linearly with the increase of the concentrations of the corresponding template molecule in the range of 0.08-20μM. The limit of detection for NE and E were 9 and 12 nM (3σ/K), respectively.