Metal Nanoparticle-utilized technologies for biosensing applications

Abstract

Over the past decades, there is a strong drive towards developments of simple, convenient, and cost-effective protocols that are suitable for point-of-care-testing (POCT) in facility-limited environments. Colorimetric and fluorescent biosensing technologies have been widely used in clinical applications, such as biological imaging and sensing.$^1$ With the aim of achieving rapid, accurate, and cost-effective diagnosis, the study of novel nanomaterial in biological applications is highly demanded. Moreover, the drive to measure trace amount of biological targets imposes the new demands of the fluorescent signaling materials with increased photostability and quantum yields.$^2$ The desired signaling materials are those which are bright, photostable and amenable to conjugate biomolecules.

In chapter 2, we describe a rapid, sensitive and selective colorimetric sensing platform for biothiols in human serum, which relies on the biothiols induced aggregation of silver nanoprims (AgNPRs) and the anti-etching effect of biothiols. The developed assay utilized AgNPR, which possess a three-dimensional anisotropic structure, as a key detection component for the colorimetric detection of biothiols. The diagnostic capability and potential in practical applications of this method were demonstrated by determining biothiols in human serum. In principle, biothiols bind to the AgNPRs surface through Ag-S covalent interactions and protect the AgNPRs from etching by human serum. Thus, inhibited the “blue to yellow” color transition. Furthermore, by tuning the pH of the reaction buffer, homocysteine (Hcy) and cysteines (Cys) can be discriminated from other biothiols in human serum by observing Hcy and Cys induced aggregation of AgNPRs under different pH conditions. Based on this simple and cost-effective strategy, we determined Hcy as low as $0.028 \muM$ at pH 7 with the linear detection range from $0.5-7 \muM$ in 1% human serum. We also determined Hcy and Cys as low as $0.056 \muM and 0.022 \muM$ at pH 5 with the linear detection range from $0-9 \muM and 3-8 \muM$, respectively in 3% human serum. While reduced glutathione (r-GSH) did not induce the aggregation of AgNPRs under all pH conditions (5-10). Additionally, due to the anti-etching effect of r-GSH, it protects AgNPRs in human serum through the covalent bonding formed by the thiol group in r-GSH and AgNPRs surface. These results indicated that our strategy is suitable for rapid and convenient identification of biothiols in human serum samples.

In chapter 3, we developed a new ultrasensitive colorimetric AChE assay that exploits both aggregation and morphology transformation of silver nanoprisms (AgNPRs) with the aim of overcoming the limitations of conventional AChE assays. We believe this newly developed method is of great importance for the study of Alzheimer disease (AD) and AChE inhibitor screening.

In chapter 4, We described europium ion ($Eu^{3+}$) doped silver-silica core-shell nanoparticles ($AgNP@SiO_2$) as a rapid and selective sensing platform for the detection of tetracycline (Tc) in tap water, which relies on the energy transfer from Tc to $Eu^{3+}$ and the metal-enhanced fluorescence (MEF) of the EuTc fluorescence complex. A rationally designed $AgNP@SiO_2$ has been synthesized considering the proper metal core size, the distance between the metal core and the fluorophore, and the plasmon band overlap between the $AgNP@SiO_2$ and the EuTc complex in order to achieve sufficient fluorescence enhancement. Based on this rationally designed MEF system, 28 fold of fluorescent intensity enhancement of EuTc were observed and Tc was reliably determined down to 83.1nM. Furthermore, practical applicability of this strategy was successfully demonstrated by detecting Tc in tap water. This work highlights the advantages of the metal-silica core-shell nanoparticles as the suitable substrates for the MEF based biosensing applications.