257
than fluorescence dyes [139]. Noble metals were used mainly in colorimetric and
surface-enhanced Raman spectroscopy (SERS) sensing. Colorimetric assays are fast
and simple. The signal transduction relies on the color change of the nanoparticle
suspension due to different interparticle distances or aggregation states [152]. It has
been studied for the detection of DNA, diagnosis of pathogen infection, and pathogen monitoring in water samples. However, the aggregation state of nanoparticles is
sensitive to the solution chemistry and difficult to control. The coexisting water/
wastewater constituents will greatly affect results, reducing reproducibility. The
SERS phenomenon is attributed to both electromagnetic effect and chemical mechanisms related to the charge transfer between the noble metals and the target molecules [231]. As a result, the efficiency of Raman scattering can be enhanced more
than 10
14
-fold, which is even capable of detecting a single molecule [246].
Silica nanoparticles doped with either organic or inorganic luminescent dyes
have been developed for ultrasensitive sensors. The large number of dye molecules
confined in a single silica particle guarantees huge improvement in sensitivity.
Moreover, the silica matrix protects the dye molecules from the external environment, largely suppressing photobleaching and photodegradation. The outstanding
photostability makes dye-doped silica nanoparticles especially advantageous for
applications that require high-intensity or prolonged excitations [337]. The rich
silica chemistry (e.g., silane chemistry) also helps future surface modification and
conjugation.
The high conductivity along the length makes CNTs outstanding electrode materials. As a result, CNTs can greatly facilitate electrochemical detection by promoting electron transfer and electrode-analyst interactions [219]. They have been
incorporated into electrodes via random or aligned coating, or used as a single CNT
electrode [343]. Semiconducting CNTs can be used in nanoscale field-effect transistor [118]. Besides their excellent electronic properties, the high adsorption capacity
of CNTs increases detection sensitivity [66]. The major challenge for CNT-based
sensors is the heterogeneity of CNTs. Separation of metallic and semiconducting
SWNT has been extensively studied but is still far from perfect. The production and
purification processes of CNTs often introduce impurities, contaminants, and even
degradation of the CNT structure. Therefore, better synthesis, purification, and separation are required to produce more homogeneous CNTs. Although most of these
nanosensors possess excellent photostability and sensitivity, nonspecific binding is
still a major challenge for their application in water and wastewater. Strategies to
reduce nonspecific binding and prevent undesired nanoparticle aggregation are in
critical need.
Trace Contaminant Detection
In trace organic or inorganic contaminant detection, nanomaterials can be used in
both concentration and detection. CNTs have great potential for environmental
analysis of trace metal or organic pollutants as they offer high adsorption capacity
and recovery rate as well as fast kinetics as discussed above. The pre-concentration
Current and Potential Applications for Water and Wastewater Treatment
than fluorescence dyes [139]. Noble metals were used mainly in colorimetric and
surface-enhanced Raman spectroscopy (SERS) sensing. Colorimetric assays are fast
and simple. The signal transduction relies on the color change of the nanoparticle
suspension due to different interparticle distances or aggregation states [152]. It has
been studied for the detection of DNA, diagnosis of pathogen infection, and pathogen monitoring in water samples. However, the aggregation state of nanoparticles is
sensitive to the solution chemistry and difficult to control. The coexisting water/
wastewater constituents will greatly affect results, reducing reproducibility. The
SERS phenomenon is attributed to both electromagnetic effect and chemical mechanisms related to the charge transfer between the noble metals and the target molecules [231]. As a result, the efficiency of Raman scattering can be enhanced more
than 10
14
-fold, which is even capable of detecting a single molecule [246].
Silica nanoparticles doped with either organic or inorganic luminescent dyes
have been developed for ultrasensitive sensors. The large number of dye molecules
confined in a single silica particle guarantees huge improvement in sensitivity.
Moreover, the silica matrix protects the dye molecules from the external environment, largely suppressing photobleaching and photodegradation. The outstanding
photostability makes dye-doped silica nanoparticles especially advantageous for
applications that require high-intensity or prolonged excitations [337]. The rich
silica chemistry (e.g., silane chemistry) also helps future surface modification and
conjugation.
The high conductivity along the length makes CNTs outstanding electrode materials. As a result, CNTs can greatly facilitate electrochemical detection by promoting electron transfer and electrode-analyst interactions [219]. They have been
incorporated into electrodes via random or aligned coating, or used as a single CNT
electrode [343]. Semiconducting CNTs can be used in nanoscale field-effect transistor [118]. Besides their excellent electronic properties, the high adsorption capacity
of CNTs increases detection sensitivity [66]. The major challenge for CNT-based
sensors is the heterogeneity of CNTs. Separation of metallic and semiconducting
SWNT has been extensively studied but is still far from perfect. The production and
purification processes of CNTs often introduce impurities, contaminants, and even
degradation of the CNT structure. Therefore, better synthesis, purification, and separation are required to produce more homogeneous CNTs. Although most of these
nanosensors possess excellent photostability and sensitivity, nonspecific binding is
still a major challenge for their application in water and wastewater. Strategies to
reduce nonspecific binding and prevent undesired nanoparticle aggregation are in
critical need.
Trace Contaminant Detection
In trace organic or inorganic contaminant detection, nanomaterials can be used in
both concentration and detection. CNTs have great potential for environmental
analysis of trace metal or organic pollutants as they offer high adsorption capacity
and recovery rate as well as fast kinetics as discussed above. The pre-concentration
Current and Potential Applications for Water and Wastewater Treatment