Almost all other reactants were analytical-grade reagents and were applied without further more purification. assay == 1 ) Introduction == Gold nanoparticles (AuNPs) founded colorimetric assays have been generally applied in many different research domains due to the increased extinction agent (35 orders placed of size higher than the ones from organic absorb dyes molecules [1]) and the completely unique size-dependent optic property of AuNPs [2, two to three, 4, 5 various, 6, 7]. Such strategies are very good in that they might require very simple test handling strategies and lowest instrumental expenditure and can be executed in the field with portable equipment. The realistic design of the chemistry of AuNPs can easily promote certain interactions amongst the receptors and analytes and, as a result, Oxcarbazepine makes the measurements highly picky [8]. Magnetic allergens (MPs) are likewise one of the most well-liked materials that have been widely exploited in medical diagnostics, controlled drug release and separation technologies [9, 10]. Even before the term nanotechnology was popular, iron oxide (Fe3O4) nanoparticles were used to magnetically isolate and purify proteins, DNA, viruses and even whole mammalian cells [11]. Recently, bifunctional Au-Fe3O4nanoparticles, recognized as a major advancement in nanobiotechnology generating, from the two components, excellent surface chemistry, special optical properties, and superparamagnetic properties, have been synthesized by various strategies [12, 13, 14, 15, 16, 17, 18], and many applications in a wide variety of fields have been found, such as DNA hybridization assay and protein detection [19, 20, 21, 22, 23]. Dopamine (DA) is a monoamine neurotransmitter distributed in the central neural system brain tissues and body fluids of mammals. It plays pivotal roles in the function of the central nervous, renal, hormonal and cardiovascular system. Detection of DA is important in diagnoses, monitoring, prevention and treatment of some certain diseases, such as Parkinsons disease, Alzheimers disease, Huntingtons disease, epilepsy, pheochromocytoma and neuroblastoma [24]. Herein, we reported a sandwich-type strategy for DA detection using magnetic Fe3O4particles and dithiobis(sulfosuccinimidylpropionate) (DTSSP)-modified AuNPs (DTSSP-AuNPs). Oxcarbazepine This method is based on the following two facts: (1) DA can form Mmp23 stable, robust anchor onto the surface of iron oxide by the interaction of catechol and iron, facilitating the conjugation of biomolecules onto the Fe3O4surface via the amine coupling reaction between DA and biomolecules [25, 26, 27, 28, 29, 30]; and (2) the amine-reactive succinimidyl residues on the surface of AuNPs can react selectively with the primary amine group of DA [4, 31]. As a result, decrease in the UV/Vis absorbance of DTSSP-AuNPs suspension was observed in the external magnetic field when DA and Fe3O4were successively added to the DTSSP-AuNPs suspension. The method is sensitive and selective to DA detection without the requirement of expensive and complicated instruments. To the best of our knowledge, this is the first magnetic Fe3O4-based biosensor with AuNPs as colorimetric probes. == 2 . Experimental Section == == 2 . 1 . Reagents and Materials == Dithiobis(sulfosuccinimidylpropionate) (DTSSP) was purchased from Chemsky International Co., Ltd. (Shanghai, China), dihydroxyphenylacetic acid (DOPAC), glutamic acid, tyrosine and trisodium citrate were obtained from Sigma-Aldrich (Shanghai, China). Norepinephrine and dopamine hydrochloride were obtained from Aladdin reagent Inc. (Shanghai, China). All of other reagents were analytical-grade reagents and were used without further purification. DA and norepinephrine were prepared with HCl solution (pH 2) and diluted to the desired concentration with phosphate buffer solution (PBS, 2 mM, pH 7. 2) containing 0. 1 mM Na2SO3. The other small-molecule organic chemicals were dissolved in deionized water. Unless otherwise noted, the reactions were conducted at room temperature. == 2 . 2 . Synthesis of Oxcarbazepine DTSSP-AuNPs == The citrate-stabilized AuNPs were prepared using a trisodium citrate reduction method. The DTSSP-modified AuNPs were prepared by ligand-exchange reaction between DTSSP and citrate-stabilized AuNPs, as in previous reports [4, 31, 32]..