(85e) Colorimetric Detection of Nitrite Ions Based on the Aggregation of Gold Nanoparticles

Many bacteria associated with urinary tract infections (UTIs) produce enzymes that reduce nitrate to nitrite. After the presentation of clinical symptoms, UTI diagnosis is based on a point-of-care (POC) test, the urine dipstick. Nitrite detection is a key component of these POC tests. However, interference, variable capacity of bacteria to reduce nitrites, diet, or dwell time of urine in the bladder likely lead to lower reliability and accuracy of nitrite testing with diagnostic sensitivity being reported between 10 and 93%. Although urine cultures or advanced characterization methods, such as chemiluminescence, electrochemistry, and surface-enhanced Raman scattering, are more definitive in diagnosis, they are slow tests that could lead to the deterioration of patients from delayed treatment. For this reason, POC testing remains an important diagnostic step in UTI management. Here we describe new, simple and highly sensitive colorimetric assays that provide visual on-site analysis. These facile colorimetric assays are based on the anti-aggregation of gold nanoparticles. The gold nanoparticles are first coated with functional ligands that cause an immediate color change to blue, which is associated with widespread aggregation of the nanoparticles. After the addition of nitrite ions, the nanoparticles are re-dispersed in solution, yielding a bright red or purple color that is similar to the initial nanoparticle suspension. By varying the concentration of the ligands and nanoparticle size, the sensitivity of these nitrite sensors is greatly increased, achieving order of magnitude improvements in detection limits. Furthermore, the effect of this assay on synthetic urine has been explored to understand the effect of foreign ions on the detection of nitrite.