An important consideration for the development of biosensors is the adsorption of the biorecognition element to the surface of a substrate. As the first step in the immobilization process, adsorption affects most immobilization routes and much attention is given into the research of this process to maximize the overall activity of the biosensor. The use of nanomaterials, specifically nanoparticles and nanostructured films, offers advantageous properties that can be fine-tuned to maximize interactions with specific proteins to maximize activity, minimize structural changes, and enhance the catalytic step. In the biosensor field, protein-nanomaterial interactions are an emerging trend that span across many disciplines. This review addresses recent publications about the proteins most frequently used, their most relevant characteristics, and the conditions required to adsorb them to nanomaterials. When relevant and available, subsequent analytical figures of merits are discussed for selected biosensors. The general trend amongst the research papers allows concluding that the use of nanomaterials has already provided significant improvements in the analytical performance of many biosensors and that this research field will continue to grow.

This paper describes the development and use of a handheld and lightweight stamp for the production of microfluidic paper-based analytical devices (μPADs). We also chemically modified the paper surface for improved colorimetric measurements. The design of the microfluidic structure has been patterned in a stamp, machined in stainless steel. Prior to stamping, the paper surface was oxidized to promote the conversion of hydroxyl into aldehyde groups, which were then chemically activated for covalent coupling of enzymes. Then, a filter paper sheet was impregnated with paraffin and sandwiched with a native paper (n-paper) sheet, previously oxidized. The metal stamp was preheated at 150 °C and then brought in contact with the paraffined paper (p-paper) to enable the thermal transfer of the paraffin to the n-paper, thus forming the hydrophobic barriers under the application of a pressure of ca. 0.1 MPa for 2 s. The channel and barrier widths measured in 50 independent μPADs exhibited values of 2.6 ± 0.1 and 1.4 ± 0.1 mm, respectively. The chemical modification for covalent coupling of enzymes on the paper surface also led to improvements in the colour uniformity generated inside the sensing area, a known bottleneck in this technology. The relative standard deviation (RSD) values for glucose and uric acid (UA) assays decreased from 40 to 10% and from 20 to 8%, respectively. Bioassays related to the detection of glucose, UA, bovine serum albumin (BSA), and nitrite were successfully performed in concentration ranges useful for clinical assays. The semi-quantitative analysis of all four analytes in artificial urine samples revealed an error smaller than 4%. The disposability of μPADs, the low instrumental requirements of the stamp-based fabrication, and the improved colour uniformity enable the use of the proposed devices for the point-of-care diagnostics or in limited resources settlements.

A systematic investigation was conducted to study the effect of paper type on the analytical performance of a series of microfluidic paper-based analytical devices (μPADs) fabricated using a CO₂ laser engraver.