Fast Detection of Bacterial Contamination

Summary of the paper "Engineering Nanostructured Porous SiO2 Surfaces for Bacteria Detection via Direct Cell Capture" by Naama Massad-Ivanir, Giorgi Shtenberg, Adi Tzur, Maksym A. Krepker, and Ester Segal. From Analytical Chemistry. http://pubs.acs.org/doi/pdfplus/10.1021/ac200407w

Fast detection of bacterial contamination in food and water could prevent much disease and mortality. Most current techniques rely on cell culture, PCR, or ELISA assays which can be time consuming and need to be performed in a lab. The researchers in this paper have developed an optical label-free biosensing platform for bacteria detection based on nano-structured, oxidized porous silicon (pSiO2). This device captures bacterial cells on its surface by means of antibodies (lgG) attached to the pSi surface. When a bacterium is bound to the surface, it induces changes in the optical interference spectrum of the biosensor.

pSi is a form of silicon full of nanoporous holes which impart the element with a huge surface to volume ratio. Discovered by accident at Bell labs in the 1950’s, the discovery was forgotten for years until it was found that the porous silicon displays quantum confinement effects, and can emit photons upon excitation. The refractive index (RI) of pSi is determined by the degree of porosity and RI of the substance inside the pores. The large surface area allows for easy adherence by organic molecules, and the substance is quite biocompatible, breaking down into nontoxic silicic acid. Si wafers are electrochemically etched in HF and ethanol, and then thermally oxidized in a furnace. The porosity and dimensions of the pSiO2 layers are found by using spectroscopic liquid infiltration. A reflectance spectrum of the porous film is taken in air and immersed in different liquids with varying Rls.

This device captures bacterial cells on its surface by means of antibodies (lgG) attached to the pSi surface. When a bacterium is bound to the surface, it induces changes in the optical interference spectrum of the biosensor. Upon exposure to a target molecule or cell, the media in the pores is replaced by the analyte, causing a shift in the refractive index of the film. This biosensor design can be used for detecting DNA, proteins, bacteria and enzyme activity. It also does not require cell lysis, as in previous optical biosensors. The scientists demonstrated the applicability of their biosensors in detection of low bacteria concentration as low as 10^4 cells/ ml for E. coli K12, with a response time of several minutes.

Porous Silicon.

This is an interesting paper that holds promise for real-time detection of bacterial contamination, and could be useful for food and water quality control. Though the paper claims that their sensor can detect bacteria other than E. coli, no work was shown to prove this point. The authors mentioned the cost and tedious fabrication necessary for other biosensors, but they did not compare these factors with their own process. Their detection limit was 10^4 cells/ mL, while the other biosensors they discussed offer more sensitive detection down to 10^2 cells/ mL. More work should be performed to improve the sensitivity of their technique.