Thermally Carbonized Porous Silicon for Robust Label-Free DNA Optical Sensing.

In this work, thermal carbonization is shown to provide the necessary surface passivation to enable highly robust DNA detection on a porous silicon (PSi) platform, overcoming previous corrosion challenges with detection of negatively charged biomolecules. The stability of thermally carbonized PSi (TCPSi), oxidized PSi (OPSi), and undecylenic acid-modified PSi (UAPSi) is compared in phosphate-buffered saline and during DNA sensing experiments. Reflectance measurements reveal an improvement in stability and DNA sensor response for TCPSi compared to OPSi and UAPSi. TCPSi exhibits a large positive sensor response with >90% DNA hybridization efficiency. In comparison, UAPSi shows a smaller positive DNA sensor response, likely lessened by a small corrosion effect, while OPSi exhibits a large negative sensor response, indicating significant induced PSi corrosion that confounds the ability of OPSi to yield meaningful readouts of DNA hybridization events. This work expands the application of TCPSi for its more widespread usage in sensing applications where competing substrate corrosion may influence device stability.

Photonic crystal nanobeam biosensors based on porous silicon.

Photonic crystal (PhC) nanobeams (NB) patterned on porous silicon (PSi) waveguide substrates are demonstrated for the specific, label-free detection of oligonucleotides. These photonic structures combine the large active sensing area intrinsic to PSi sensors with the high-quality (Q) factor and low-mode volume characteristic of compact resonant silicon-on-insulator (SOI) PhC NB devices. The PSi PhC NB can achieve a Q-factor near 9,000 and has an approximately 40-fold increased active sensing area for molecular attachment, compared to traditional SOI PhC NB sensors. The PSi PhC NB exhibits a resonance shift that is more than one order of magnitude larger than that of a similarly designed SOI PhC NB for the detection of small chemical molecules and 16-base peptide nucleic acids. The design and fabrication of PSi PhC NB sensors are compatible with CMOS processing, sensor arrays, and integration with lab-on-chip systems.

DNA-Hybridization Detection on Si(100) Surfaces Using Light-Activated Electrochemistry: A Comparative Study between Bovine Serum Albumin and Hexaethylene Glycol as Antifouling Layers.

Light can be used to spatially resolve electrochemical measurements on a semiconductor electrode. This phenomenon has been explored to detect DNA hybridization with light-addressable potentiometric sensors and, more recently, with light-addressable amperometric sensors based on organic-monolayer-protected Si(100). Here, a contribution to the field is presented by comparing sensing performances when bovine serum albumin (BSA) and hexaethylene glycol (OEG6) are employed as antifouling layers that resist nonspecific adsorption to the DNA-modified interface on Si(100) devices. What is observed is that both sensors based on BSA or OEG6 initially allow electrochemical distinction among complementary, noncomplementary, and mismatched DNA targets. However, only surfaces based on OEG6 can sustain electroactivity over time. Our results suggest that this relates to accelerated SiO x formation occasioned by BSA proteins adsorbing on monolayer-protected Si(100) surfaces. Therefore, DNA biosensors were analytically explored on low-doped Si(100) electrodes modified on the molecular level with OEG6 as an antifouling layer. First, light-activated electrochemical responses were recorded over a range of complementary DNA target concentrations. A linear semilog relation was obtained from 1.0 × 10-11 to 1.0 × 10-6 mol L-1 with a correlation coefficient of 0.942. Then, measurements with three independent surfaces indicated a relative standard deviation of 4.5%. Finally, selectivity tests were successfully performed in complex samples consisting of a cocktail mixture of four different DNA sequences. Together, these results indicate that reliable and stable light-activated amperometric DNA sensors can be achieved on Si(100) by employing OEG6 as an antifouling layer.

Connecting electrodes with light: one wire, many electrodes.

The requirement of a wire to each electrode is central to the design of any electronic device but can also be a major restriction. For example it entails space restrictions and rigid device architecture in multi-electrode devices. The finite space that is taken up by the array of electrical terminals and conductive pads also severely limits the achievable density of electrodes in the device. Here it is shown that a travelling light pointer can be used to form transient electrical connections anywhere on a monolithic semiconductor electrode that is fitted with a single peripheral electrical terminal. This is achieved using hydrogen terminated silicon electrodes that are modified with well-defined organic monolayers. It is shown that electrochemical information can be either read from or written onto these surfaces. Using this concept it is possible to form devices that are equivalent to a conventional electrode array but that do not require a predetermined architecture, and where each element of the array is temporally "connected" using light stimulus; a step change in capability for electrochemistry.

A robust DNA interface on a silicon electrode.

Two different interfaces prepared via UV-hydrosilylation of undecylenic acid and 1,8-nonadiyne on silicon(111) have been explored to develop a robust electrochemical DNA sensor. Electrodes modified with undecylenic acid were found to stably immobilise DNA but could not resist the growth of insulating oxides, whereas 1,8-nonadiyne modified electrodes satisfy both requirements.

The rapid formation of functional monolayers on silicon under mild conditions.

We report on an exceedingly mild chemical functionalization of hydrogen-terminated Si(100) with unactivated and unprotected bifunctional α,ω-dialkynes. Monolayer formation occurs rapidly in the dark, and at room temperature, from dilute solutions of an aromatic-conjugated acetylene. The method addresses the poor reactivity of p-type substrates under mild conditions. We suggest the importance of several factors, including an optimal orientation for electron transfer between the adsorbate and the Si surface, conjugation of the acetylenic function with a π-system, as well as the choice of a solvent system that favors electron transfer and screens Coulombic interactions between surface holes and electrons. The passivated Si(100) electrode is amenable to further functionalization and shown to be a viable model system for redox studies at non-oxide semiconductor electrodes in aqueous solutions.

The detailed characterization of electrochemically switchable molecular assemblies on silicon electrodes.

In this paper we explore a multi-step synthetic strategy toward fabrication of monolayer-modified Si(100) electrodes that can be electrochemically switched. The synthetic scheme is modular and benefits from an established intramolecular lactonization scheme of benzoquinone analogs. A redox-tagged pendant group can be released from the surface such as to allow for in situ monitoring of the switch process. We show that this model system can be used to elucidate chemical and structural events for a surface dynamic system that is rapidly gaining popularity. The influence of polarization times, overpotentials and semiconductor doping type on the kinetic of the switch event is also investigated. In both basic and acidic aqueous electrolytes the release of suitable redox-active markers is found to require unexpectedly large cathodic overpotentials. The release event is accompanied by minor oxidation of the electrode surface and the switched constructs can be regenerated by chemical means with no appreciable deterioration of surface quality.