No abstract

Chemical sensors are molecular receptors that transform their chemical information into analytically useful signals upon binding to specific guests. These sensors are attracting attention owing to their potential for easy detection and quantification of the pollutant species in many fields of application, such as waste management, environmental chemistry, clinical toxicology, and bioremediation of radionuclides.[1-6] Among these, the sensitive detection of heavymetal ions, such as mercury and lead, is critical for monitoring the environment as they are highly toxic and common environmental pollutants. Although instrumental analyses such as atomic absorption or atomic emission spectroscopy are currently used in applications relevant to the detection of toxic metal ions, there is still a need to develop inexpensive and easy methods for the detection of these toxic ions. In view of these sophisticated experimental methods, emphasis is currently being placed on the development of sensor materials for the detection of toxic ions that offer high sensitivity,

With recent advances in mesostructured materials and nanotechnologies, new methods are emerging to design optical sensors and biosensors, and to develop highly sensitive solid sensors. Here, highly sensitive, low cost, simple nanosensor designs for naked‐eye detection of toxic metal ions are successfully developed by the immobilization of commercially available α,β,γ,δ‐tetrakis(1‐methylpyridinium‐4‐yl)porphine p‐toluenesulfonate (TMPyP) and diphenylcarbazide (DPC), and chemically synthesized 4‐n‐dodecyl‐6‐(2‐thiazolylazo) resorcinol (DTAR) and 4‐n‐dodecyl‐6‐(2‐pyridylazo) phenol (DPAP) chromophore molecules into spherical nanosized cavities and surfaces. A rational strategy was crucial to develop optical nanosensors that can be used to control accurate recognition and signaling abilities of analyte species for ion‐sensing purposes. This is the first reported evidence of the significant key factors of the development of receptors as ‘indicator dyes' and surface‐confinement materials as ‘carriers' to broadening the applicability of optical chemical sensors for selective discrimination of trace levels of toxic analytes. In all the nanosensor design techniques presented here, a dense pattern of immobilized hydrophobic ‘neutral' and hydrophilic ‘charged' chromophores with intrinsic mobility, as a result of extremely robust constructed sequences on nanoscale structures, is a key to enhancing the sensing functionality of optical nanosensors. These nanosensor designs can be used as cage probe sinks with reliable control, for the first time, over the colorimetric recognition of cadmium ions to low levels of concentration in the range of 10–9 to 10–10 M. Optimization of control sensing conditions is established to achieve enhanced signal response and color intensities. These chemical nanosensors are reversible and have the potential to serve effectively in on‐site field analysis of environmental samples, which eliminates the necessity for instrument‐dependent analysis. Moreover, these new classes of optical cage sensors exhibit long‐term stability of signaling and recognition functionalities that in general provide extraordinary sensitivity, selectivity, reusability, and fast kinetic detection and quantification of various deleterious metal ions in our environment.

With recent advances in materials science and nanotechnology, development of optical chemosensors with uniformly shaped three-dimensional (3D) nanostructures applicable for large-scale sensing systems of toxic pollutants can forge new frontiers in materials. Here, highly ordered cubic Fd3m silica monoliths that had nanopore-like cages were fabricated, for the first time, by direct templating of cationic surfactant phases. This simple strategy offered significant control over the pore connectivity and structural regularity of the cubic Fd3m geometry. The potential functionalities of these uniformly sized cage cubic Fd3m materials show promise as the primary component in efficient sensing systems that can satisfy analytical needs as well, such as simplicity in fabrication design and sensing functionality in terms of selectivity and sensitivity with a fast response time of the recognition of pollutant cations. However, successful immobilization of chromophore probe molecules into the 3D network matrixes enabled manipulation of optically defined chemosensors into new shapes and functionality for visual detection of toxic analytes. Here, 3D cubic Fd3m chemosensors were developed and fabricated and successfully enabled highly revisable, selective and sensitive detection of Bi-(III) target ions down to nanomolar concentrations (∼10 -10 mol/dm 3 ) with rapid response assessment (e25 s). Significantly, the HOM nanosensors not only worked under standardized conditions but also could be used for reliable sensing of the Bi(III) ion in a real-life sample such as wastewater.

A low cost, solid optical sensor for the rapid detection of low concentrations of Hg2+ in aqueous media was prepared by the monolayer functionalization of mesoporous silica with 5,10,15,20-tetraphenylporphinetetrasulfonic acid (TPPS), anchored by N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TMAC). The detection is based on the color change of TPPS from orange to green as a result of the formation of a charge-transfer complex with Hg2+. The intensity of the charge-transfer band varies linearly with Hg2+ in the concentration range from zero to 2.5 x 10(-7) mol dm(-3). The lower detection limit observed for Hg2+ concentration is 1.75 x 10(-8) mol dm(-3). The material exhibits good chemical and mechanical stability, and did not show any degradation of TPPS for a period of eight months. The sensor was applied for the analysis of various environmental samples. The effects of pH, sample volume, reaction time, amount of material, and the presence of foreign ions on the detection method are discussed.