9. Gold and Silver Fluorescent Nanomaterials as Emerging Probes for Toxic and Biochemical Sensors

A high percentage of deaths per year worldwide are caused by environmental pollution. It is well known that excessive usage of toxic chemicals including heavy metal ions, pesticides, other food toxins, etc. leads to adverse effect to living organisms and contributing to biodiversity losses and severe damage to the environment. Thus, the detection of toxic compounds with high sensitivity and specificity in real time is essential nowadays. For the past few decades, the development of chemical sensors have received much attention due to the sensitivity and selectivity achieved, possibility of in situ monitorization with rapid response, low cost, simple instrumental setup, etc. Traditionally, the use of organic dyes such as cyanine, fluorescein, etc. or more recently the use of semiconductor quantum dots and upconverting nanoparticles has been employed as fluorophores in order to generate optical sensors. However, these fluorophores have certain limitations such as poor photostability, large particle size, or poor water solubility. On the other hand, metal nanoclusters (NCs) and nanodots (NDs) show strong luminescence with high photostability, large Stokes shifts, and good aqueous solubility and biocompatibility. It is well known that the size of metal nanoclusters is comparable to the Fermi wavelength of electrons (∼0.7 nm), giving rise to molecular-like properties and size-dependent fluorescence from visible to near-infrared range. These novel properties have been exploited in the field of chemical and biochemical sensing, bioimaging, electronic device fabrication, clean energy storage, etc.

In this chapter, we briefly summarize the most common synthesis procedures and recent progress of luminescent Ag/AuNCs and NDs. Their application for chemical and biochemical sensing is also collected, paying special attention to the detection of toxic heavy metals (including mercury, lead, copper, chromium, arsenic, etc.), toxic ions (such as cyanide, sulfide, etc.), biological compounds (cysteine, tyrosine, cysteamine, glutathione, glucose, H₂O₂, etc.), drugs (mercaptopurine, penicillamine, clioquinol, antibiotics, etc.) and some other interesting molecules (salicylaldehyde, poly diallyldimethyl ammonium chloride, sodium dodecyl sulfate), toxic contaminants (tea polyphenols, melamine, bisphenol A, etc.), and pathogenic bacteria.