Engineering Metal Halide Perovskite Nanocrystals with BODIPY Dyes for Photosensitization and Photocatalytic Applications.

The sensitization of surface-anchored organic dyes on semiconductor nanocrystals through energy transfer mechanisms has received increasing attention owing to their potential applications in photodynamic therapy, photocatalysis, and photon upconversion. Here, we investigate the sensitization mechanisms through visible-light excitation of two nanohybrids based on CsPbBr3 perovskite nanocrystals (NC) functionalized with borondipyrromethene (BODIPY) dyes, specifically 8-(4-carboxyphenyl)-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BDP) and 8-(4-carboxyphenyl)-2,6-diiodo-1,3,5,7-tetramethyl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (I2-BDP), named as NC@BDP and NC@I2-BDP, respectively. The ability of I2-BDP dyes to extract hot hole carriers from the perovskite nanocrystals is comprehensively investigated by combining steady-state and time-resolved fluorescence as well as femtosecond transient absorption spectroscopy with spectroelectrochemistry and quantum chemical theoretical calculations, which together provide a complete overview of the phenomena that take place in the nanohybrid. Förster resonance energy transfer (FRET) dominates (82%) the photosensitization of the singlet excited state of BDP in the NC@BDP nanohybrid with a rate constant of 3.8 ± 0.2 × 1010 s-1, while charge transfer (64%) mediated by an ultrafast charge transfer rate constant of 1.00 ± 0.08 × 1012 s-1 from hot states and hole transfer from the band edge is found to be mainly responsible for the photosensitization of the triplet excited state of I2-BDP in the NC@I2-BDP nanohybrid. These findings suggest that the NC@I2-BDP nanohybrid is a unique energy transfer photocatalyst for oxidizing α-terpinene to ascaridole through singlet oxygen formation.

Linker-Assisted CdS-TiO2 Nanohybrids as Reusable Visible Light Photocatalysts for the Oxidative Hydroxylation of Arylboronic Acids.

A variety of phenols have been obtained in aqueous media with moderate to excellent chemical yields (≤100%) by using arylboronic acids and esters as substrates, a robust CdS-TiO2 nanohybrid as a heterogeneous photocatalyst, visible light irradiation (467 nm), and an O2-saturated atmosphere. The nanohybrid was prepared through a linker-assisted methodology that uses mercapto alkanoic acids as the organic linkers. The nanohybrid showed improved photocatalytic activity in the hydroxylation of substituted arylboronic acids and phenyl boronic esters compared with that of pristine CdS quantum dots. The nanohybrid can be reused in up to five photocatalytic cycles with ∼90% of its outstanding activity preserved.

Near-infrared excitation/emission microscopy with lanthanide-based nanoparticles.

Near-infrared optical imaging offers some advantages over conventional imaging, such as deeper tissue penetration, low or no autofluorescence, and reduced tissue scattering. Lanthanide-doped nanoparticles (LnNPs) have become a trend in the field of photoactive nanomaterials for optical imaging due to their unique optical features and because they can use NIR light as excitation and/or emission light. This review is focused on NaREF4 NPs and offers an overview of the state-of-the-art investigation in their use as luminophores in optical microscopy, time-resolved imaging, and super-resolution nanoscopy based on, or applied to, LnNPs. Secondly, whenever LnNPs are combined with other nanomaterial or nanoparticle to afford nanohybrids, the characterization of their physical and chemical properties is of current interest. In this context, the latest trends in optical microscopy and their future perspectives are discussed.

Ruddlesden-Popper Hybrid Lead Bromide Perovskite Nanosheets of Phase Pure n=2: Stabilized Colloids Stored in the Solid State.

Ruddlesden-Popper lead halide perovskite (RP-LHP) nano-nanostructures can be regarded as self-assembled quantum wells or superlattices of 3D perovskites with an intrinsic quantum well thickness of a single or a few (n=2-4) lead halide layers; the quantum wells are separated by organic layers. They can be scaled down to a single quantum well dimension. Here, the preparation of highly (photo)chemical and colloidal stable hybrid LHP nanosheets (NSs) of ca. 7.4 µm lateral size and 2.5 nm quantum well height (thereby presenting a deep blue emission at ca. 440 nm), is reported for the first time. The NSs are close-lying and they even interconnect when deposited on a substrate. Their synthesis is based on the use of the p-toluenesulfonic acid/dodecylamine (pTS/DDA) ligand pair and their (photo)chemical stability and photoluminescence is enhanced by adding EuBr2 nanodots (EuNDs). Strikingly, they can be preserved as a solid and stored for at least one year. The blue emissive colloid can be recovered from the solid as needed by simply dispersing the powder in toluene and then using it to prepare solid films, making them very promising candidates for manufacturing devices.

Water-Soluble Naked Gold Nanoclusters Are Not Luminescent.

Here, the synthesis of water-dispersible naked gold nanoclusters (AuNCnaked ) is reported by a simple reduction of HAuCl4 with NaOH at room temperature, and it is shown that they are non-luminescent. They are then easily passivated with different thiols and adenosine monophosphate, leading to luminescent NCs. This is an important finding because the photoluminescence of the passivated NCs can now be clearly attributed to the ligand-AuNC surface interaction. These results are also highly relevant from the point of view of the preparation of luminescent NCs from the same NC batch. This strategy can be valuable for the preparation of a broad range of nano(bio)composites.

A Metal-Free, Nonconjugated Polymer for Solar Photocatalysis.

Heterogeneous catalysts that can absorb light over the solar range are ideal for green photocatalysis. Recently, attention has been directed towards the generation of novel solar-light photocatalysts, in particular, metal-free polymers. Herein, it is demonstrated that a metal-free, nonconjugated, anthraquinone-based copolymer (poly[1,4-diamine-9,10-dioxoanthracene-alt-(benzene-1,4-dioic acid)] (COP)) with a strong absorption in the visible region is effective as a sunlight heterogeneous photocatalyst. As a proof of concept, it has been used to mineralize 2,5-dichlorophenol (2,5-DCP) in water under air and sunlight irradiation. The photocatalytic efficiency of COP compares well with that of TiO2 -P25 when the reaction is carried out in a solar photoreactor in acid medium. Steady-state and time-resolved (absorption and emission) studies performed on COP suspended in 6:4 DMF/H2 O have provided valuable information about the COP species generated under different pH conditions. Steady-state absorption and fluorescence data are consistent with the existence of a tautomeric equilibrium between the 9,10-keto and 1,10-iminoketo quinoid forms for the anthraquinone in the ground state. Moreover, in basic media, transient absorption measurements showed the presence of two bands ascribed to the tautomeric triplet excited states, whereas only one of the triplets was observed in acid medium. A mechanism for the photocatalyzed degradation of 2,5-DCP by COP is proposed on the basis of these observations.

The Luminescence of CH3 NH3 PbBr3 Perovskite Nanoparticles Crests the Summit and Their Photostability under Wet Conditions is Enhanced.

CH3 NH3 PbBr3 perovskite nanoparticles (PAD ) are prepared with a photoluminescence quantum yield of ≈100% in air atmosphere by using the quasi-spherical shaped 2-adamantylammonium bromide (ADBr) as the only capping ligand. The photostability under wet conditions of this kind of nanoparticles is enhanced by using cucurbit[7]uril-adamantylammonium (AD@CB) host-guest complexes as the capping ligand.

Organic-inorganic and all-inorganic lead halide nanoparticles [Invited].

Organic-inorganic (hybrid) and all-inorganic lead halide perovskites, in particular APbX(3) where A is an organic cation (methylammonium or formamidinium) or cesium cation and X = Cl, Br, I, respectively, are of great interest in photovoltaic devices and as luminescent materials for light-emitting devices. It has recently been demonstrated that they can be prepared not only as nanoparticulate material by using the pores of mesoporous films, but also as colloidal nanoparticles, which exhibit enhanced optical properties with respect to the bulk material. We summarize here the methods reported for their preparation as well as their optical features. Experimental and theoretical studies on this class of materials are ongoing and there is still a demand for enhancing their emissive properties, stability in polar solvents, dispersibility in different media and/or photostability.

Nontemplate synthesis of CH3NH3PbBr3 perovskite nanoparticles.

To date, there is no example in the literature of free, nanometer-sized, organolead halide CH3NH3PbBr3 perovskites. We report here the preparation of 6 nm-sized nanoparticles of this type by a simple and fast method based on the use of an ammonium bromide with a medium-sized chain that keeps the nanoparticles dispersed in a wide range of organic solvents. These nanoparticles can be maintained stable in the solid state as well as in concentrated solutions for more than three months, without requiring a mesoporous material. This makes it possible to prepare homogeneous thin films of these nanoparticles by spin-coating on a quartz substrate. Both the colloidal solution and the thin film emit light within a narrow bandwidth of the visible spectrum and with a high quantum yield (ca. 20%); this could be advantageous in the design of optoelectronic devices.

Triggering the generation of an iron(IV)-oxo compound and its reactivity toward sulfides by Ru(II) photocatalysis.

The preparation of [Fe(IV)(O)(MePy2tacn)](2+) (2, MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) by reaction of [Fe(II)(MePy2tacn)(solvent)](2+) (1) and PhIO in CH3CN and its full characterization are described. This compound can also be prepared photochemically from its iron(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru(II)(bpy)3](2+) as photosensitizer and a sacrificial electron acceptor (Na2S2O8). Remarkably, the rate of the reaction of the photochemically prepared compound 2 toward sulfides increases 150-fold under irradiation, and 2 is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound 2 with sulfides under different conditions, which demonstrated that both light and [Ru(II)(bpy)3](2+) are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [Ru(III)(bpy)3](3+) oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by 2 to the corresponding sulfoxide.

Sensitive and selective plasmonic assay for spermine as biomarker in human urine.

A simple, fast, and highly selective and sensitive colorimetric assay to detect nanomolar levels of spermine in human urine (healthy donors, cancer patients) is reported. This assay is based on the absence of a competitive organic capping on the gold nanoparticles together with the high affinity of the amine groups of the analyte for the nanoparticle surface.

Raspberry-like Nanoheterostructures Comprising Glutathione-Capped Gold Nanoclusters Grown on the Lanthanide Nanoparticle Surface.

Bare lanthanide-doped nanoparticles (LnNPs), in particular, NaYF4:Yb3+,Tm3+ NPs (UCTm), have been seeded in situ with gold cations to be used in the subsequent growth of gold nanoclusters (AuNCs) in the presence of glutathione (GSH) to obtain a novel UCTm@AuNC nanoheterostructure (NHS) with a raspberry-like morphology. UCTm@AuNC displays unique optical properties (multiple absorption and emission wavelengths). Specifically, upon 350 nm excitation, it exhibits AuNC photoluminescence (PL) (500-1200 nm, λmax 650 nm) and Yb emission (λmax 980 nm); this is the first example of Yb sensitization in a UCTm@AuNC NHS. Moreover, under 980 nm excitation, it displays (i) upconverting PL of the UCTm (at the blue, red and NIR-I, ca. 800 nm, regions); (ii) two-photon PL of AuNC; and (iii) down-shifting PL of thulium (around 1470 nm). The occurrence of energy transfer from UCTm to AuNCs in the UCTm@AuNC NHS was evidenced by the drastic lengthening of the AuNC PL lifetime (τPL) (from few hundred nanoseconds to more than one hundred microseconds). Initial biological assessment of UCTm@AuNC NHSs in vitro revealed high biocompatibility and bioimaging capabilities upon near-infrared excitation.

In situ colorimetric quantification of silver cations in the presence of silver nanoparticles.

Silver ions (Ag(+)) can be quantified in situ in the presence of AgNPs by using a colorimetric sensing probe (3,3',5,5'-tetramethylbenzidine). Interestingly, it also enables detection of the Ag(+) adsorbed on the AgNP surface. This is relevant to design new methods to make AgNPs while ensuring the total reduction of Ag(+).

Sensing chiral drugs by using CdSe/ZnS nanoparticles capped with N-acetyl-L-cysteine methyl ester.

Chiral quantum dots (QDs), differing in their core or shell size and, consequently, in their optical properties, were synthesized by the treatment of commercially available amine-capped quantum dots with methyl ester N-acetyl-L-cysteine (CysP). Interestingly, their colloidal methanol solutions remain stable for several months. Their NMR and IR spectra were in accordance with CysP binding to the QD surface through two anchoring groups; its thiolate (strongly bound) and the carbonyl group of its ester (weaker bound) group, whereas their circular dichroism (CD) spectra showed a new broad redshifted band, suggesting that the attachment to the QD surface modified the conformational equilibrium towards conformer(s) with optical activity in this region. These QDs were sufficiently fluorescent to perform studies of the chiral recognition of drugs, in particular the aryl propionic acids (APAs) ketoprofen (KP), naproxen (NP), flurbiprofen (FP), and ibuprofen (IP). We used different drug concentration ranges, depending on the QD solubility. All the assayed drugs quenched the QD emission in a concentration-dependent mode. Quenching fluorescence assays with the chiral QDs (CS@CysP) showed their extraordinary capacity for the chiral recognition of KP, NP, and FP, and particularly in the case of KP and FP, a remarkable positive allosteric effect was detected for the R enantiomer. By using a drug/CS@CysP molar ratio of 5000:1 and 2500:1, the changes of intensity and the sign of the CD spectrum of the drug evidenced the dissociation of the drug carboxylic group in the presence of the QD.

Unzipping nucleoside channels by means of alcohol disassembly.

Gold nanoparticles capped with simple adenosine derivatives can form colloidal aggregates in nonpolar solvents. Theoretical calculations indicate the formation of organic channels by the supramolecular assembly of the nanoparticles by means of hydrogen bonds between the adenine moieties. The aggregates were only negligibly sensitive to nPrOH, iPrOH, and tBuOH, whereas some showed a similar response to MeOH and EtOH, and others showed high selectivity toward MeOH. DNA nucleoside derivatives (1-(2-deoxy-ß-D-ribofuranosyl)-5-methyluracil and 2',3'-O-isopropylideneadenosine) as well as thymine and other aromatic compounds such as pyrene derivatives (pyrene, 1-chloropyrene, 1-hydroxypyrene, (1-pyrenyl)methanol, and 2-hydroxynapthalene) did not induce disassembly of the nanoparticle aggregates. Data suggest that the nucleoside channels allow access to alcohols according to their size, and an efficient interaction between the alcohol and the adenine units destabilizes the hydrogen bonds, which eventually leads to nanoparticle disassembly.

Orthogonal functionalisation of upconverting NaYF4 nanocrystals.

A simple and straightforward method for the orthogonal functionalisation of upconverting NaYF4 nanocrystals (UCNCs)-doped withYb(3+) and Er(3+)-based on N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide/N-hydroxysuccinimide (EDC/NHS) selective reactions between two dyes and two different reactive groups present at the periphery of the upconverting nanocrystals is reported. Organic-soluble UCNCs of 10 and 50 nm in size are encapsulated efficiently in a 1:1 mixture of two commercial 3000 Da poly(ethylene glycol) derivatives with two different reactive groups (amino and carboxylic groups). The water-dispersible UCNCs are non-cytotoxic, stable in the physiological environment, and present free amine and carboxylic reactive groups on their periphery, allowing rapid, selective, and modular covalent conjugation to payloads through EDC/NHS reactions. PEG-encapsulated UCNCs with and without covalent conjugation to payloads are characterised in vitro through spectroscopic, dynamic light scattering, and electron microscopy measurements. Living cell analyses coupled with TEM measurements confirm the uptake and low cytotoxicity of the coated UCNCs. They are linked covalently to two different dyes, internalised by living cells, and analysed by confocal microscopy. The related colocalisation measurements prove the reactivity of both amines and carboxylic acids on the periphery of the nanocrystals. This approach demonstrates that it is possible to produce water-dispersible and cyto-compatible dual-functional UCNCs.

Synergistic or antagonistic effect of lanthanides on Rose Bengal photophysics in upconversion nanohybrids?

A nanohybrid made of a xanthenic dye, rose bengal, grafted to an ytterbium and erbium codoped upconversion nanoparticle (UCNP) served as a proof-of-concept to evaluate the fundamental mechanisms which govern the dye photophysics upon interaction with the UCNP. Both photoactive lanthanides strongly influence the singlet and triplet excited states of rose bengal.

Photocatalysis Based on Metal Halide Perovskites for Organic Chemical Transformations.

Heterogeneous photocatalysts incorporating metal halide perovskites (MHPs) have garnered significant attention due to their remarkable attributes: strong visible-light absorption, tuneable band energy levels, rapid charge transfer, and defect tolerance. Additionally, the promising optical and electronic properties of MHP nanocrystals can be harnessed for photocatalytic applications through controlled crystal structure engineering, involving composition tuning via metal ion and halide ion variations, dimensional tuning, and surface chemistry modifications. Combination of perovskites with other materials can improve the photoinduced charge separation and charge transfer, building heterostructures with different band alignments, such as type-II, Z-scheme, and Schottky heterojunctions, which can fine-tune redox potentials of the perovskite for photocatalytic organic reactions. This review delves into the activation of organic molecules through charge and energy transfer mechanisms. The review further investigates the impact of crystal engineering on photocatalytic activity, spanning a diverse array of organic transformations, such as C-X bond formation (X = C, N, and O), [2 + 2] and [4 + 2] cycloadditions, substrate isomerization, and asymmetric catalysis. This study provides insights to propel the advancement of metal halide perovskite-based photocatalysts, thereby fostering innovation in organic chemical transformations.

The Dark Side of Lead-Free Metal Halide Nanocrystals: Substituent-Modulated Photocatalytic Activity in Benzyl Bromide Reduction.

We illustrate here the high photocatalytic activity of sustainable lead-free metal halide nanocrystals (NCs), namely, Cs3Sb2Br9 NCs, in the reduction of p-substituted benzyl bromides in the absence of a cocatalyst. The electronic properties of the benzyl bromide substituents and the substrate affinity to the NC surface determine the selectivity in C-C homocoupling under visible light irradiation. This photocatalyst can be reused for at least three cycles and preserves its good performance with a turnover number of ca. 105,000.

Governing the emissive properties of 4-aminobiphenyl-2-pyrimidine push-pull systems via the restricted torsion of N,N-disubstituted amino groups.

Donor-acceptor-substituted biphenyl derivatives are particularly interesting model compounds, which exhibit intramolecular charge transfer because of the extent of charge transfer between both substituents. The connection of a 4-[1,1'-biphenyl]-4-yl-2-pyrimidinyl) moiety to differently disubstituted amino groups at the biphenyl terminal can offer push-pull compounds with distinctive photophysical properties. Herein, we report a comprehensive study of the influence of the torsion angle of the disubstituted amino group on the emissive properties of two pull-push systems: 4-[4-(4-N,N-dimethylaminophenyl)phenyl]-2,6-diphenylpyrimidine (D1) and 4-[4-(4-N,N-diphenylaminophenyl)phenyl]-2,6-diphenylpyrimidine (D2). The torsion angle of the disubstituted amino group, either N,N-dimethyl-amine or N,N-diphenyl-amine, at the biphenyl end governs their emissive properties. A drastic fluorescence quenching occurs in D1 as the solvent polarity increases, whereas D2 maintains its emission independently of the solvent polarity. Theoretical calculations on D1 support the presence of a twisted geometry for the lowest energy, charge-transfer excited state (S1,90), which corresponds to the minimum energy structure in polar solvents and presents a small energy barrier to move from the excited to the ground state, thereby favoring the non-radiative pathway and reducing the fluorescence efficiency. In contrast, this twisted structure is absent in D2 due to the steric hindrance of the phenyl groups attached to the amine group, making the non-radiative decay less favorable. Our findings provide insights into the crucial role of the substituent in the donor moiety of donor-acceptor systems on both the singlet excited state and the intramolecular charge-transfer process.