This work quantifies the impact of steric crowding on whole antibody (Ab) receptor immobilization and target Ab detection and also demonstrates how the versatile biotin/streptavidin receptor immobilization system must be tuned to optimize target detection in designing biosensors. Results are demonstrated on a label-free optical biosensor comprised of n-type macroporous porous silicon (PSi) with ∼88-107 nm diameter pores. We employ a sandwich assay scheme comprised of a linking chemistry (biotin/streptavidin) to attach biotinylated anti-rabbit IgG (receptor) to detect rabbit IgG (target). A "bottoms-up" approach was taken to investigate each layer of the sandwich assay to optimize target binding. Steric crowding was observed to hinder subsequent layer binding for each layer in the sandwich (biotin, streptavidin, and receptor). Our results give definitive evidence that onset of steric crowding within the biotin layer occurs at a surface coverage of 57% which is much higher compared to published work on well ordered self-assembled biotin monolayers on planar gold surfaces. This difference is attributed to the topographical heterogeneity of the PSi substrate. Streptavidin binding to surface-linked biotin was altered by preblocking streptavidin binding sites with biotin. Through consistent trends in data, preblocking SA was shown to reduce steric crowding within the SA layer, which translated into increased receptor immobilization. The final detection range of rabbit IgG was 0.07-3 mg ml -1 (0.23-9.8 μg mm -2 ) and binding specificity was demonstrated employing anti-chicken IgG control receptor. This study underlines the importance of considering binding avidity and surface topography in optimizing chip-based biosensors.

Organic-inorganic hybrids with controlled morphology at the nanometer scale represent an exciting class of materials that can display unique properties that are culminated by the characteristics of each building block. Recent research highlights their potential as biomimetic composites and application in biosensing, lab-on-chip devices, drug delivery and tissue engineering. Here we focus on the emerging class of biomaterials that integrate polymers with nanostructured porous silicon and emphasize the design of advanced 'smart' functions. Porous silicon is an appealing biomaterial due to the ease of tuning its many attractive properties, including pore morphology, photonic properties, biocompatibility, biodegradation and surface chemistry. An overview is presented of the principle concepts of design and fabrication of porous silicon-polymer hybrids. Current achievements in biomedical applications are reviewed and future prospects and challenges for healthcare technologies are discussed.

The future of rapid point-of-care diagnostics depends on the development of cheap, noncomplex, and easily integrated systems to analyze biological samples directly from the patient (e.g. blood, urine, and saliva). A key concern in diagnostic biosensing is signal differentiation between non-specifically bound material and the specific capture of target molecules. This is a particular challenge for optical detection devices in analyzing complex biological samples. Here we demonstrate a porous silicon (PSi) label-free optical biosensor that has intrinsic size-exclusion filtering capabilities which enhances signal differentiation. We present the first demonstration of highly repeatable, specific detection of immunoglobulin G (IgG) in serum and whole blood samples over a typical physiological range using the PSi material as both a biosensor substrate and filter.

The incorporation of a chemo-responsive hydrogel into a 1D photonic porous silicon (PSi) transducer is demonstrated. A versatile hydrogel backbone is designed via the synthesis of an amine-functionalized polyacrylamide copolymer where further amine-specific biochemical reactions can enable control of cross-links between copolymer chains based on complementary target–probe systems. As an initial demonstration, the incorporation of disulfide chemistry to control cross-linking of this hydrogel system within a PSi Bragg mirror sensor is reported. Direct optical monitoring of a characteristic peak in the white light reflectivity spectrum of the incorporated PSi Bragg mirror facilitates real-time detection of the hydrogel dissolution in response to the target analyte (reducing agent) over a timescale of minutes. The hybrid sensor response characteristics are shown to systematically depend on hydrogel cross-linking density and applied target analyte concentration. Additionally, effects due to responsive hydrogel confinement in a porous template are shown to depend on pore size and architecture of the PSi transducer substrate. Sufficient copolymer and water is removed from the PSi transducer upon dissolution and drying of the hydrogel to induce color changes that can be detected by the unaided eye. This highlights the potential for future development for point-of-care diagnostic biosensing.

Currently, there is need for laboratory based high-throughput and reliable point-of-care drug screening methodologies. We demonstrate here a chip-based label-free porous silicon (PSi) photonic sensor for detecting opiates in urine. This technique provides a cost-effective alternative to conventional labeled drug screening immunoassays with potential for translation to multiplexed analysis. Important effects of surface chemistry and competitive binding assay protocol on the sensitivity of opiate detection are revealed. Capability to tune sensitivity and detection range over ∼3 orders of magnitude (18.0 nM -10.8 μM) was achieved by varying the applied urine specimen volume (100 -5 μl), which results in systematic shifts in the competitive binding response curve. A detection range (0.36 -4.02 μM) of morphine in urine (15 μl) was designed to span the current positive cut-off value (1.05 μM morphine) in medical opiate urine screening. Desirable high crossreactivity to oxycodone, in addition to other common opiates: morphine, morphine-3-glucuronide, 6-acetyl morphine demonstrates an advantage over current commercial screening assays, while low interference with cocaine metabolite was maintained. This study uniquely displays PSi sensor technology as an inexpensive, rapid, and reliable drug screening technology. Furthermore, the versatile surface chemistry developed can be implemented on a range of solid-supported sensors to conduct competitive inhibition assays.