Miniaturized systems, such as integrated microarray and microfluidic devices, are constantly being developed to satisfy the growing demand for sensitive and highthroughput biochemical screening platforms. Owing to its recyclability, and robust mechanical and optical properties, poly(methyl methacrylate) (PMMA) has become the most sought after material for the large-scale fabrication of these platforms. However, the chemical inertness of PMMA entails the use of complex chemical surface treatments for covalent immobilization of proteins. In addition to being hazardous and incompatible for large-scale operations, conventional biofunctionalization strategies pose high risks of compromising the biomolecular conformations, as well as the stability of PMMA. By exploiting radio frequency (RF) air plasma and standard 1ethyl-3-(3-(dimethylamino)propyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) chemistry in tandem, we demonstrate a simple yet scalable PMMA functionalization strategy for covalent immobilization (chemisorption) of proteins, such as green fluorescent protein (GFP), while preserving the structural integrities of the proteins and PMMA. The surface density of chemisorbed GFP is shown to be highly dependent on the air plasma energy, initial GFP concentration, and buffer pH, where a maximum GFP surface density of 4 × 10 −7 mol/m 2 is obtained, when chemisorbed on EDC−NHS-activated PMMA exposed to 27 kJ of air plasma, at pH 7.4. Furthermore, antibody-binding studies validate the preserved biofunctionality of the chemisorbed GFP molecules. Finally, the coupled air plasma and EDC−NHS PMMA biofunctionalization strategy is used to fabricate microfluidic antibody assay devices to detect clinically significant concentrations of Chlamydia trachomatis specific antibodies. By coupling our scalable and tailored air plasma-enhanced PMMA biofunctionalization strategy with microfluidics, we elucidate the potential of fabricating sensitive, reproducible, and sustainable high-throughput protein screening systems, without the need for harsh chemicals and complex instrumentation.
Silver (Ag) is one of the most sensitive noble metals for nanoplasmonic applications. However, Ag is prone to oxidation in wet environments (e.g., aqueous and organic solvents), which usually leads to poor adhesion between Ag and dielectrics, thereby limiting its use in biosensing applications. To address this challenge, we propose a new design principle where one metal corrodes preferentially when in electrical contact with the other metal, in the presence of an electrolyte. Specifically, by using a simple two-step thermal dewetting protocol, we fabricate bimetallic Ag and titanium nanoislands (Ag/Ti NI) on top of SiO 2 surfaces, where Ti suppresses the Ag from its oxidation in wet environments. As the number of free electrons in valence band increases from 1 in Ag to 5 in Ag/Ti, our bimetallic Ag/Ti NI achieve a superior refractive index sensitivity of 112 nm/RIU in comparison to Ag-based plasmonic materials with similar sizes (20−40 nm) and morphologies (i.e., nanoisland shapes), with high stability (100 days). We further demonstrate the use of Ag/Ti NI substrates as a generic refractive index sensor and for the detection of human C-reactive protein with a limit of detection of 80 fM. Our work presents a very promising synthesis strategy to prevent oxidation issues of metals at the nanoscale, which is crucial in developing nanomaterial based biosensors with a long shelf life.
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