Over the last decade, researchers have paid more attention to gold nanoparticles (AuNPs) due to their unique optical properties that can be used for a nanosensor platform based on colorimetry and fluorescence detection. AuNPs are developed via functionalization as a sensing element based on colorimetric or fluorimetric detection for the target analyte, such as metal ions in water samples. In this chapter, we have focused on novelties of the functionalized AuNPs as sensors for Al3 + ion detection. The novel sensing approaches were discussed using functionalized AuNPs to enhance the possibility of developing nanosensors for detection of Al3 + ions that exhibit desirable characteristics, such as high sensitivity, selectivity, rapid, and simple. The recent advantages of functionalized AuNPs that highlight the reliability of their applications for detection of trace Al3 + ions in water samples are evaluated.

A novel colorimetric method based on polyaniline (PANI) film for the development of smart packaging, as a chemical sensor for real-time monitoring of the microbial breakdown products in the headspace of packaged fish is described. This on-package indicator contains PANI film, that responds through visible color change to a variety of basic volatile amines (specifically known as total volatile basic nitrogen (TVBN)) released during fish spoilage period. The PANI film characteristics and its response to TVBN were studied. A kinetic approach was used to correlate the ammonia response of the PANI film to that of the fish spoilage. Color changes, in terms of total color difference of PANI, correlated well with TVBN levels of fish. Apart from TVBN, trials on milkfish sample (Chanos chanos) have verified that the PANI film response also correlates well with microbial growth patterns in fish samples, especially the changing microbial populations (total viable count (TVC) and Pseudomonas spp.). These responses enabled the real-time monitoring of fish spoilage either at various constant temperatures or with temperature fluctuations. The PANI film can be recycled several times using an acid solution to regenerate the PANI surface. Thus, PANI film can be considered as a low-cost sensor suitable for smart packaging applications.

In order to develop simple sensing system for pork adulteration in processed meats, the DNA biosensor based on 20 nm gold nanoparticles (AuNPs) with aggregation property has been developed as a simple colorimetric detection of target DNA. Adsorption of single-stranded (ss) DNA on AuNPs protects the particles against salt-induced aggregation. However, mixing and annealing of a 27-nucleotide (nt) ssDNA probe on AuNPs with denatured DNA of different processed meats differentiated between perfectly matched and mismatch hybridization at a critical annealing temperature (55 °C). The AuNPs change color from red to purple, in 10 mM phosphate buffer saline (PBS). At a hybridizing temperature (55 °C), non-target mismatched DNA provided hybridization products allowing probe to be free and adsorbed to AuNPs. This prevented AuNPs from salt-induced aggregation as the color still red. In matched DNA, hybridization would occurred, allowing probe to be occupied. This facilitated AuNPs from salt-induced aggregation and induced colorimetric change of particles from red to purple. These signals could be observed easily with naked eye. This label-free DNA nanobiosensor should find applications in food analysis and other DNA based screening.