A gold nanoparticle based colorimetric test allied to Salmonella LAMP amplification: toward the revolution of Food Quality Control

Abstract

Microorganisms are the primary cause of food spoilage and foodborne illness. Every year Salmonella causes 1.3 billion cases of disease worldwide, representing a considerable burden in both developing and developed countries. Despite of great advances in technologies in the last decade, current Salmonella detection methods are not fast and reliable enough. In order to address these issues a test based in the colorimetric properties of gold nanoparticles was combined with LAMP technique for the detection of Salmonella. Gold nanoparticles were synthesized and functionalised with thiol-modified DNA oligonucleotides (Gold nanoprobes) able to recognize the target sequence - ttrRSBCA locus from Salmonella. It was amplified by LAMP technique, which was optimized in this study. Four batches of gold nanoprobe were synthesized from two different batches of gold nanoparticles and under different pH. The arising gold nanoprobes showed some differences between them, namely in their stability and their detection capacity. These differences were investigated and it was verified that their cause was related to the conditions of functionalisation, namely ultrasound, ionic strength and nanoparticles size. The last was confirmed with TEM. Au-nanoprobes were then used in a colorimetric method for DNA detection based on a non-cross-linking approach - upon increasing ionic strength, Au-nanoprobes aggregate and the solution changes colour from the original red to blue, due to the red-shift of the typical Surface Plasmon Resonance (SPR) band of gold nanoparticles; the presence of a complementary target to the probe sequence, prevents aggregation of the Au-nanoprobes and the solution remains red. Gold nanoprobes detection capacity was tested with LAMP and PCR products and it was verified that a better discrimination is achieved with PCR products. The system developed here presents some advantages but still needs optimisation for further application in pathogen detection in food.