In the above test, alternative arylamine and SEAr substrates have been studies in place of
3 and
7, but these have been found to be optimal [
9]. The selectivity of the Griess test for nitrite over other anions derives from the fact that only nitrite is able to diazotize an arylamine under acidic conditions. Thus, other analytes, in particular nitrate ions, give a negative result. This key concept of nitrite-induced diazotization has been exploited in other nitrite probe designs. Gold nanoparticles have been surface-functionalized with covalently linked variants of
3 and
6—in the absence of nitrite, the colloidal nanoparticles are red due to a surface plasmon resonance, but addition of nitrite results in diazotization and crosslinking of the nanoparticles through the newly formed azo bonds, leading to aggregation and precipitation, and hence loss of color [
10]. An alternative approach employing gold nanorods involves surface immobilization of arylamines, which are protonated under the acidic assay conditions, leading to a high positive surface charge. Nitrite induced diazotization followed by reduction (i.e., overall deamination) leads to neutral surface groups, and hence to a decreased surface charge. This in turn leads to aggregation and colour change [
11]. Further variants employing gold nanostructures have also been described [
12–
14], as well as a system employing silver nanoparticles [
15]. In addition, colorimetric nitrite probes that comprise aryl 1,2-diamine groups have been reported, the diazotization of an amine here leading to the formation of a benzotriazole instead of an azo linkage [
16]. In the case of 2-arylethynylaniline colorimetric nitrite probes, diazotization leads to formation of a different heterocycle, 4(1H)-cinnolones [
17,
18]. An aza-BODIPY chromophore with conjugated arylamine substituents is reported to show a colorimetric response to nitrite as a result of diazotization [
19]. Pyrrole has also been reported to be a colorimetric probe for nitrite, acting by a mechanism other than amine diazotization, although the exact mechanism was not disclosed [
20]. Autocatalytic approaches to increase the sensitivity of colorimetric nitrite sensors have been developed [
21]. Elsewhere, organometallic complexes have also been reported as colorimetric probes for nitrite [
22,
23].