Biosensors are very important in society and have many applications, including detecting biological hazards and diagnosing certain diseases. Nanoparticles, with their unique size-dependent properties, are an extremely promising technology for biosensor creation. However, some of the most promising models employ aqueous unsupported nanoparticles, which are difficult to adapt to applications outside of a laboratory. Instead, by supporting the nanoparticles in a gel, our biosensor would be portable, more adaptable to different environments, and more commercially applicable than unsupported, liquid-based biosensors.

Our biosensor consists of gold nanoparticles (AuNP's) embedded in a supporting polymer matrix, which permits the development of portable devices with potential for gas phase operation. The supporting matrix consists of poly(ethylene glycol) diacrylate (PEGDA) hydrogels, in which particles can be easily embedded. Biosensor detection is based on the color shift experienced by AuNP's, which is a function of their surface plasmon resonance. AuNP's less than 40 nm appear red, and as they grow larger the visible color shifts to violet. By bringing two AuNP close together, this same affect can be achieved as a result of proximity. If the particles are moved apart again, the absorbance shifts back resulting in a red solution. Our biosensor is designed to detect trypsin, a common protease used to decrease cell binding, and also important in diagnosis of cystic fibrosis. To produce violet particle aggregates, a peptide (N term-CGGGRGDSGGGC-COOH term) is used to bind the AuNPs together. If trypsin is present it will cleave the peptide, returning the particles to their initial red unaggregated state.

AuNP's were successfully aggregated in solution using the peptide, resulting in a color change. The absorbance spectra of the AuNP-peptide solution shifted so the system appears violet, whereas the unaggregated AuNP absorbance peak did not move. However, the system did not function properly in biological solution as a result of unwanted AuNP aggregation caused by saline and other solution contaminants. To reduce aggregation, we decided to use a stabilizing agent that would protect the AuNP's but still allow the peptide to bind. After experimenting with several agents, we are currently using tri(ethylene glycol) mono-11-mercaptoundecyl ether. This ligand is mixed together with the peptide and the two are then added simultaneously to the AuNP's. This is yielding a better passivated AuNP system than before, but the proper ratio of stabilizing agent to peptide has not yet been achieved.

Next, we plan to suspend these conjugates in PEGDA hydrogels and confirm that a color change can be observed in the presence of sensate (trypsin). This biosensor is designed to detect the extent of cell cleavage in response to trypsin. However, this biosensor could be customized to almost any enzyme-substrate system. Any substrate with thiol ends (which can be added through cysteine termination) has the ability to bind the AuNPs together, and any substrate specific enzyme can cleave the peptide bond activating the sensor. With the enzyme-substrate flexibility and the adaptability that our sensor will offer, it could be customized to fit almost any sensing need and would therefore have large commercial appeal.