The diatom silicified cell wall (frustule) contains pore arrays at the micro- to nanometer scale that display efficient luminescence within the visible spectrum. Morphometric analysis of the size and arrangement of pores was conducted to observe whether any correlation exists with the photoluminescence (PL) of two diatom species of different ages. UV-excited PL displays four clearly defined peaks within the blue-region spectrum, on top of the broad PL characteristic of synthetic porous silicon dioxide, recorded for reference and where discrete lines are absent. A set of shifted emission lines was observed when diatom cultures reached adulthood. These discrete line shifts correlate with structural changes observed in adult frustules: reduction in pore diameter; appearance of pores within pores, 10 nm in size; an increase in the gap distance between stria; and the deposition of several girdle bands with a concomitant increase in the diatom waist length, as well as the appearance of pores on such bands. Destruction of the pores results in the disappearance of all discrete emission lines. The PL shifts are correlated with a substantial increment of Si-OH groups adsorbed on the frustule surface, as revealed by Fourier transform infrared spectroscopy.

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A SSW/Al-Cu formed from an industrial solid waste and Al-Cu Nps are utilized for the removal of fluoride from aqueous solutions. The SSW/Al-Cu was obtained by a chemical reduction method. The SSW/Al-Cu was characterized by TEM, SEM, FT-IR, XRD, BET, and pH techniques. The Nps were formed as bimetallic oxides and deposited in the form of spheroidal particles forming agglomerations. The sizes of these particles range from 1 to 3 nm. The surface area and average pore width of SSW/Al-Cu were 2.99 m/g and 17.09 nm, respectively. The adsorption kinetics were better described using the second-order model, pointing to chemical adsorption with an equilibrium time of 540 min. The thermodynamic parameters obtained here confirm the spontaneous and endothermic nature of the process. The percentage of fluoride removal was 89.5% using the four-bladed disk turbine, and computational fluid dynamics (CFD) modeling demonstrated that using the four-bladed disk turbine helped improve the fluoride removal process. The maximum adsorption capacity was 3.99 mg/g. The Langmuir-Freundlich model best describes the adsorption process, which occurred by a combination of mechanisms, such as electrostatic interactions between the ions involved in the process. This study proves that the chemical modification of this waste solid created an efficient bimetallic nanomaterial for fluoride removal. Furthermore, the method of preparation of these nanocomposites is quite scalable.

Diatom frustules have species‐specific patterns of pores, striae, pores, and nanopores, periodically arranged on its silica surface, as sets of cavities that modify the vacuum electromagnetic density of states. Therefore, frustules may be considered photonic crystals; the interaction with light‐emitting sources inside the pores may potentially result in enhancement or inhibition of their spontaneous radiative emission rate and frequencies. In this work, we studied the photoluminescence of cadmium sulfide nanoparticles (CdS‐NP) deposited inside frustule cavities that conveyed evidence of cavity–NP interaction. We synthesized CdS‐NP, a semiconductor compound achieving quantum dots small enough to impose confinement effects to the electronic states. CdS‐NP and their clusters were physiosorbed onto the surface, striae, and predominantly inside the pores of the cleansed frustules of Amphora sp. A broad peak with a maximum intensity at 437 nm (2.84 eV) was recorded after excitation with a 375 nm light source, showing a large blue shift and signal amplification of the CdS‐NP photoluminescence when these were embedded inside the pores of the silica frustule. Using the Brus equation, we estimated a NP size of 4.1 ± 0.2 nm for the CdS‐NP snuggly packed inside the smaller pores of the frustule, of 10 ± 0.7 nm in average diameter, The emission Purcell enhancement factor for an emitting atom in a cavity was calculated. The obtained Q factor (c. 5) was smaller than typical Q factors for designed semiconductor cavities of similar dimensions, an expected situation if it is assumed that the pores are open‐ended cavities.