Gold Nanoparticles: Bigger and Better
Use of gold nanoparticles/nanostructures for biosensing application is exploding and that will necessarily drive need for strict control over the size and shape of these particles. Until now it is easy to get monodisperse gold nanoparticles of diameter of 2-10 nm using sodium borohydride reduction method and 12-50nm using sodium citrate reduction. Now a new paper in JACS is reporting a new method for synthesis of highly monodisperse gold nanoparticles of diameters upto 200nm. Method uses hydroquinone driven reduction of gold ions in presence of seed gold nanoparticles. Gold nanoparticles can be easily modified using thiol polyethylene glycol.
Gold Nanoparticle Based Scanometric Immunoassay
Continuing on the theme of growing gold nanoparticles with electroless gold deposition, Chad Mirkin’s group is reporting using similar approach to further improve the sensitivity of their scanometric multiplexed immunoassay. Scanometric immunoassay method uses traditional sandwich immunoassay where detection antibody is labeled with gold nanoparticles. After binding of detection antibodies, the gold nanoparticles are grown by electroless deposition of silver hence increasing the scattering intensity. To further improve the sensitivity, the silver enhancement step has been replaced by electroless gold deposition. The improved sensitivity comes from larger particle size- 420nm, 1400nm and 2700nm after one, two, and three round of gold deposition. Whereas, for the silver enhancement method the sizes are 100, 270, and 550nm after one, two, and three cycles.
Using gold enhancement method, the sensitivity for PSA (prostrate specific antigen) was 300aM in buffer compared to 30fM using silver enhancement.
Paper Based Biosensors
Whitesides group is reporting on a new, inexpensive handheld spectrophotometer for quantitative measurement of color change of their µPAD (Paper based Analytical Devices) biosensor. Quantitative measure of proteins is important when different level of proteins reflects different diseases. This technology is part of their continuing effort to provide a cost effective, portable, rugged, adaptable, and easy to use diagnostic devices for locations with limited access to physicians or paramedics. Their earlier work on µPAD includes 3-D microfluidic devices using paper and using camera phones for reading and transmitting data from µPAD.
Another report uses good old lateral flow type technology to look for inhibitors of acetylcholine esterase (AChE) inhibitors on paper based biosensor. All the reagents are integrated on the sensor and the response can be read by eye. Several pesticides could be detected in low nanomolar range within 5 minutes and with minimal matrix effect.