Nanomateirals As Enzyme Mimetics And Its Application In Analysis

It was well known that natural enzymes have been found extensive applications in variousfields because they can catalyze chemical reactions specifically and efficiently under mildconditions. Unfortunately, the catalytic activity of the natural enzymes are sensitive toenvironmental conditions and can easily be denatured and digested. Furthermore, thepreparation, purification and storage of natural enzymes are usually time-consuming andexpensive. Thus, artificial enzyme mimics come to be of great interest. Since the pioneeringwork of Yan et al using Fe3O4nanocrystals as peroxidase-mimetics, different nanostructuresincluding metal oxide nanomaterial, carbon nanomaterial, noble metal nanoparticles and othernanomaterials were also found to possess peroxidase-like activity which can catalyze theoxidation of the typical chromogenic substrates by H2O2. In comparison with natural enzyme,enzyme mimetics based on nanostructures own several advantages including controlledsynthesis in low-cost, high catalytic activity, and high stability against stringent conditions. So,nanomaterials are promising candidates as enzyme mimetics.(1) Mercury(II) ion enabled citrate-capped silver nanoparticles (Cit-AgNPs) to exhibitcatalytic activity toward the oxidation of the typical chromogenic substrates3,3’,5,5’-tetramethylbenzidine (TMB) by dissolved oxygen under mild conditions, suggestinga new type of oxidase mimics. In addition, the oxidase-like activity of Cit-Ag NPs wassensitive to the concentration of mercury(II) ion and selective to mercury(II) ion among theother metal ions. Hg2+was reduced on the surface of Cit-AgNPs to form Hg-Ag alloys. theHg-Ag alloys activated oxygen and generated superoxide anions, which oxidized TMB.Based on the relationship between the absorbance of652nm after oxidation and theconcentration of mercury(II), a facile colorimetric mercury(II) ion sensor was developed.Under the optimum conditions, the range of the method for Hg2+determination was1.0×10-7to1.0×10-5mol/L and the detection limit was28nmol/L.(2) We found that silver halides (AgX, X=Cl, Br, I) nanoparticles (NPs) possessed dualresponsive enzyme mimetic activities. In the presence of hydrogen peroxide, AgX NPscatalyzed the oxidation of the typical substrates of peroxidase, exhibiting peroxidase-likeactivity. Under visible-light (λ≥420nm) stimulation, the AgX NPs also demonstrated thesame enzyme-like activity in the absence of H2O2. It should be emphasized that thephoto-stimulated AgI NPs showed surprisingly high enzyme-like activity over a broad pHrange (3.0-7.0) even at neutral pH. In addition, the photostimulated AgI NPs demonstratedenzyme-like activity in the absence of the destructive hydrogen peroxide. A catalyticmechanism was found for the enzyme-like activity of CS-AgI NPs in the presence ofhydrogen peroxide or visible light activation. The photoactivated CS-AgI NPs possessingenzyme-like activity were applied as robust nanoprobes for sensitive, selective, and fastcolorimetric detection of cancer cells.(3) Under visible-light (λ≥400nm) stimulation, CS-GO nanocomposite can catalyze theoxidation of the typical substrates of TMB, exhibiting enzyme mimetic activities. The optimalpH is4.0and an optimal temperature is30℃. Typical Michaelis-Menten curves can be obtained in a certain range of TMB concentration, the Michaelis–Menten constant (Km) wasobtained by using Lineweaver–Burk plot is32.6μmol/L, indicating that CS-GO has a higheraffinity for TMB. At the same time, the catalytic mechanism was found for the enzyme-likeactivity of CS-GO nanocomposite under visible-light (λ≥400nm) stimulation, thephoto-generated holes (h+) are main reactive species responsible for TMB oxidation. Based onthe competition between glucose and CS for ConA and the enzyme-like activity of CS-GO todevelop a new colorimetric nanoprobe to determine glucose. The range of the method forglucose determination was2.5×10-6to5×10-3mol/L and the detection limit was0.5μmol/L.