Neeraj Kumar Joshi

The United Nations and the World Health Organization provide clear guidelines to ensure water security for urban and rural populations. Common contaminants include bacteria and a variety of organic contaminants, such as medications and agricultural runoff. The rapid advancement of point-of-use water treatment is crucial to align with these international recommendations. While some problems are chronic and require long-term solutions, others are transient contamination issues that occur without warning and frequently lead to boil water advisories that can last for extended periods. In these cases, providing reliable water security requires solutions that can be deployed rapidly, are affordable, and can be implemented at the point of use with minimal operator training. Our research explores the state of the art in photocatalysis as a method for purifying water from organic contaminants and bacteria. We present a comparative analysis of various catalysts, supports, and light sources, along with our perspective on the benefits of flow systems. Practical solutions require flow techniques that are portable and can address at least the recommended survival requirements of ~7.5 L per capita per day for small communities, schools, or small hospitals. In this perspective, we propose that flow-compatible modified TiO₂ catalysts can offer practical solutions implemented with either solar light or LED sources in the UVA or visible region.

The kinetics of the interaction of several cannabinoids with singlet oxygen have been examined in acetonitrile solution using laser flash photolysis techniques and using xanthone as a singlet oxygen sensitizer. Typical rate constants for these Type 2 reactions are in the 1–7 × 10 ⁷ M ⁻¹ s ⁻¹ range and are in accordance with typical reactivities for phenolic compounds. Protection of the phenolic function reduces the reactivity by approximately a factor of five, thus confirming that singlet oxygen quenching is dominated by the phenolic group.

We report absolute rate constants for the reaction of triplet decatungstate ( ³ DT*) with hydroperoxides using laser techniques and taking advantage of its recently discovered NIR phosphorescence.

Contaminated drinking water is a major health hazard in large urban areas as well as remote communities. Several pollutants detected in rivers and lakes are hormone disruptors that are harmful to consumers as well as aquatic life. In this contribution, we present a new material, synthesized using novel green technologies, designed for solar- or LED-driven degradation of pollutants. This material is based on a glass fiber support, loaded with black TiO₂, a modified form of TiO₂ with strong visible light absorption and without any toxic metal or non-metal dopants. This photocatalyst is fully compatible with flow applications. The effectiveness of the catalyst is demonstrated with crocin and 17β-estradiol, the former being a natural carotenoid used as a screening tool and the latter being a common hormonal disruptor. Our work shows that under visible light illumination, our supported black TiO₂ can degrade these water contaminants with greater efficiency than conventional TiO₂. We envision that our findings can contribute to the production of inexpensive, large-scale solar or LED-based water decontamination systems that could be rapidly deployed to sites in need. Operation of such systems would require minimal training and could be monitored remotely. In addition to the catalyst’s non-toxicity and inflow compatibility, the material also has a long shelf life and is easy and inexpensive to produce, making it an attractive candidate for developing water treatment devices.

The characterization of the triplet state of decatungstate ( ³ DT*) and its NIR phosphorescence with lifetimes ∼100 ns in acetonitrile allow the easy determination of rate constants that are key to understanding its role in catalysis.

In the present study, we have reinvestigated photochemistry of 2,5‐dihydroxy benzoic acid (DHBA) in a series of organic solvents of different polarities and the aqueous solution of varying pH from 1 to 14. The investigation was carried out using steady‐state and nanosecond (ns) time‐domain fluorescence measurement techniques at room temperature. Some hitherto not explored spectral features with new insight on different forms of DHBA are reported. It is found that DHBA exists as monoanionic and neutral forms in various polar protic, aprotic, and nonpolar solvents. In aqueous solution (pH 1 to 14), neutral, monoanionic, and dianion forms are found to be responsible for the observed optical spectral profiles. The observed significant Stokes shift in emission is attributed to species undergoing excited‐state intramolecular proton transfer (ESIPT) in different solvents. The analysis presented in this work elucidates and validate the presence of various species and conformers of DHBA.

Dimeric and multimeric aggregates of a model metalloporphyrin, zinc tetraphenylporphyrin (ZnTPP), have been produced in a controlled manner by incrementally increasing the water content of dilute aqueous ethanol solutions. Steady state absorption, fluorescence emission, and fluorescence excitation spectra have been measured to identify the aggregates present as a function of solvent composition. The dynamics of the excited states of the aggregates produced initially by excitation in the Soret region have been measured by ultrafast fluorescence upconversion techniques. Only the monomer produces measurable emission from S₂ with a picosecond lifetime; all Soret-excited aggregates, including the dimer, decay radiationlessly on a femtosecond time scale. The S₁ state is the only significant product of the radiationless decay of the S₂ state of the excited monomer, and the aggregates also produce substantial quantum yields of S₁ fluorescence when initially excited in the Soret region. The resulting fluorescent aggregates all decay on a subnanosecond time scale, likely by a mechanism that involves dissociation of the excited monomer from the excitonic multimer. The ZnTPP dimers excited at their ground state geometries in the Soret region exhibit a dynamic behavior that is quite different from those produced following noncoherent triplet–triplet annihilation under the same conditions. The important implications of these observations in determining the aggregation conditions promoting efficient photon upconversion by excitonic annihilation in a variety of media are thoroughly discussed.

Spectroscopic, photophysical and computational studies designed to expose and explain the differences in the efficiencies of non-coherent photon upconversion (NCPU) by triplet–triplet annihilation (TTA) have been carried out for a new series of alkyl-substituted diphenyl and tetraphenyl zinc porphyrins, both in fluid solution and in solid films. Systematic variations in the alkyl-substitution of the phenyl groups in both the di- and tetraphenyl porphyrins introduces small, but well-understood changes in their spectroscopic and photophysical properties and in their TTA efficiencies. In degassed toluene solution TTA occurs for all derivatives and produces the fluorescent S₂ product states in all cases. In PVA matrices, however, none of the di-phenylporphyrins exhibit measurable NCPU whereas all the tetraphenyl-substituted compounds remain upconversion-active. In PVA the NCPU efficiencies of the zinc tetraphenylporphyrins vary significantly with their steric characteristics; the most sterically crowded tetraphenyl derivative exhibits the greatest efficiency. DFT-D computations have been undertaken and help reveal the sources of these differences.