Study on the Sensitivity of Functionalized Nanowires Using Varied Chemicals

Abstract

Silicon nanowire-based metal-oxide-semiconductor field-effect transistors (SiNW MOSFETs) have been demonstrated excellent sensitivity and stability after surface modification and functionalization of nanowires. Chemical molecules have been applied to functionalize the surface of silicon surface. Silane coupling agents are good candidates for forming self-assembled monolayers (SAMs) by chemically interacting with silicon oxide. Those chemically modified SAMs can provide a functional surface to further conjugate biomolecules on SiNW MOSFETs. After functionalization, SiNW MOSFETs with tunably biocompatible surface can sustain a functional biointerface for biological tests. In this work, SiNW MOSFETs were fabricated using the standard I-line stepper of MOS semiconducting process and then visualized by scanning electron microscopy (SEM). The n-type SiNW MOSFETs devices were fabricated after the process of trimming, the scale of nanowire was down to a level of approximate 165 nm. 3-aminopropyl trimethoxysilane (APTMS) and 3-mercaptopropyl trimethoxysilane (MPTMS) SAMs were independently used to modify the surface of SiNW MOSFETs for pH sensing in biological buffer solution. Atomic force microscopy (AFM) and electron spectroscopy for chemical analysis (ESCA) were applied to characterize before and after surface modification. AFM found APTMS and MPTMS were successfully modified on silicon substrates. The average vertical length of APTMS and MPTMS SAMs from our AFM observation was around 2.628 nm and 2.698 nm, respectively. ESCA showed the specifically functional amino (-NH2) groups and mercapto (-SH) groups on each APTMS and MPTMS modified silicon substrates. The specific amine functional group at 399.4 eV occurred after the modification of APTMS on silicon substrate in N1s spectra. S2p spectra showed the specific binding at 163.6 eV (C-SH) and 165.8 eV (-C-S-S-C-) after the modification of MPTMS on silicon substrate. Those disulfide bonds further influenced the organization of MPTMS-SAM on the surface; therefore, the APTMS had better SAM performance on our silicon substrate. On the other hand, electrical measuring system was used for elucidating that the suitable surface modification would have great impact on the sensing response and sensitivity. Varied biological PBS solutions at different pH values showed that unmodified SiNW MOSFETs were sensitive to the H^+ ion change. When the pH level of the solution increased, the drain current of the unmodified SiNW MOSFETs decreased accordingly. In comparison with unmodified nanowires in current measurement, the changes of current of APTMS or MPTMS modified nanowires were enhanced in sensing of different pH solutions. Our results also showed that amino and mercapto groups of APTMS and MPTMS can improve the protonation and deprotonation reactions in different pH solutions. Both APTMS and MPTMS modified SiNW MOSFETs in pH sensings possessed good electrical sensing response and sensitivity in contrast with unmodified one. Moreover, in consequence of lower mercaptal groups of MPTMS on NWs, the relatively minor signal responses to varied pH solutions in MPTMS modified SiNW MOSFETs. The electrical measurement showed that the amino groups of APTMS significantly improve the sensitivity of SiNW MOSFET in different pH sensings. Our results showed that adequate modification could provide a functionable surface for SiNW MOSFETs. We inferred the APTMS modified SiNW MOSFETs could be a real-time sensor for different pH levels detection and further applied in monitoring biological environment in the future.