Staying alive: new perspectives on cell immobilization for biosensing purposes

Intact living cells, because of their simplicity of use and their ability to provide highly valuable functional information, are well suited to biosensing applications. Cells can be genetically engineered by introduction of reporter proteins, modified to achieve analyte selectivity for their sensing capabilities, and connected to a transducer to obtain whole-cell biosensors. These bioanalytical features are increasingly attracting attention in the pharmaceutical, environmental, medical, and industrial fields. Whole-cell biosensors based on different recognition elements and transduction mechanisms have been also incorporated into portable devices and, with recent advances in micro and nanofabrication and microfluidics technology, miniaturized to achieve single-cell level analysis. Cell immobilization, widely used in, for example, microbial biofermentors or bioremediation systems, is now emerging as an appealing way of integrating whole-cell biosensors into devices, to maintain long-term cell viability, to increase the reproducibility of the cell's response, and to avoid the spread of genetically modified cells into the environment, the latter being very important when devices are used for analysis in the field. A plethora of materials and functionalized surfaces have been proposed for immobilization of microbial or mammalian cells, each one having peculiar advantages and limitations. This critical review highlights and discusses recent trends, together with selected bioanalytical applications of immobilized viable cells. In particular the review focuses on some aspects that seem to hold great promise for future applications of immobilized cells, spanning from microbial biosensors to microbial biofilms, cell microarrays, and single-cell analysis.