We report the successful translation of a batch-based synthesis procedure for gold nanorods (AuNRs) to two-phase flow in a millifluidic channel, with a view to enhancing reaction control and scalability. Aqueous solutions of gold seed-particles and growth-mixture are mixed and dispersed as droplets inside a stream of organic carrier liquid, before passing through a length of gently warmed polytetrafluoroethylene (PTFE) tubing where the seeds grow into nanorods. Following inline separation of the aqueous reaction mixture from the carrier liquid, the product is collected in a solution of sodium sulphide to quench further growth of the AuNRs outside of the reactor. The size of the AuNRs is readily tuned by varying the concentration of seed particles added to each droplet and the relative flow-rates of the aqueous reagent streams, with high nanorod shape-yields of > 80 % and low length-dispersities of < 10 % being obtained under optimised conditions. In contrast to typical single-phase reactors, no issues relating to reactor fouling are encountered using the two-phase procedure deployed here, which may offer a scalable and reliable approach to shape- and size-selective synthesis of AuNRs.

Heeney et al. reflect on one of their first Materials Horizons papers (Mater. Horiz., 2014, https://doi.org/10.1039/C3MH00066D) published in the journal and discuss how their work may have influenced the research field.

Solution-processed plastic semiconductors have garnered significant attention recently due to their ease fabrication and diverse optoelectronic functionalities, positioning them as promising contenders for the next generation of semiconductors. However, comprehending the molecular ordering in polymer semiconductor blends during solution processing remains a captivating challenge. In this study, we chose poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(3-hexylselenophene-2,5-diyl) (P3HS) blends as the model system and examined the molecular ordering of blends with low molecular weights (below the entanglement) and high molecular weights (above the entanglement). By employing a combination of structural analysis, spectroscopic techniques, and theoretical modeling, valuable insight regarding the arrangement of molecules in three dimensions within P3HT/P3HS blends of varying molecular weights have been acquired. Through these analyses, we establish a comprehensive relationship between molecular weight, molecular ordering, and exciton coherence in polymer-polymer blends.

The Open Lock-In Amplifier (OLIA) is a microcontroller-based digital lock-in amplifier built from a small number of inexpensive and easily sourced electronic components. Despite its small credit card-sized form-factor and low build-cost of around US$35, OLIA is a capable instrument that offers many features associated with far costlier commercial devices. Key features include dual-phase lock-in detection at multiple harmonic frequencies up to 50 kHz, internal and external reference modes, adjustable levels of input gain, a choice between low-pass filtering and synchronous filtering, noise estimation, and a comprehensive programming interface for remote software control. OLIA comes with an optional optical breakout board that allows noise-tolerant optical detection down to the 40-pW level. OLIA and its breakout board are released here as open hardware, with technical diagrams, full parts-lists, and source-code for the firmware.

Micro- and nano-plastics (MNPs) are global contaminants of growing concern to the ecosystem and human health. In-the-field detection and identification of environmental micro- and nano-plastics (e-MNPs) is critical for monitoring the spread and effects of e-MNPs but is challenging due to the dearth of suitable analytical techniques, especially in the sub-micron size range. Here we show that thin gold films patterned with a dense, hexagonal array of ring-shaped nanogaps (RSNs) can be used as active substrates for the sensitive detection of micro- and nano-plastics by surface-enhanced Raman spectroscopy (SERS), requiring only small sample volumes and no significant sample preparation. By drop-casting 0.2-μL aqueous test samples onto the SERS substrates, 50-nm polystyrene (PS) nanoparticles could be determined via Raman spectroscopy at concentrations down to 1 μg/mL. The substrates were successfully applied to the detection and identification of ∼100-nm polypropylene e-MNPs in filtered drinking water and ∼100-nm polyethylene terephthalate (PET) e-MNPs in filtered wash-water from a freshly cleaned PET-based infant feeding bottle.

The controlled and reproducible production of high-quality nanomaterials is of paramount importance in numerous areas of science and technology. In this presentation I will describe how flow chemistry can offer a versatile and scalable approach to synthesising a broad range of nanomaterials, and how it can deliver significant improvements in control compared to conventional flask-based methods. I will focus in particular on the use of automation methods in flow chemistry and on the design of “intelligent” reactors that are capable of automatically optimising the yield or properties of a target product.

The ability to recover acetic acid and related byproducts from wastewater treatment plants would unlock a sustainable source of important building block-chemicals that are currently derived from fossil fuels. We report here a two-stage flow-based procedure for the extraction and alkaline back-extraction of acetic acid from an aqueous feed solution, using geraniol or eucalyptol as bio-derived organic solvents. In the first stage, acetic acid is extracted from the feed solution into the organic solvent; and, in the second stage, acetic acid is back-extracted into a 2-M NaOHaq solution, leaving a regenerated solvent that may be used for further extractions. Recovery efficiencies of up to 51 % and 37 % were obtained using geraniol and eucalyptol, respectively. By back-extracting acetic acid into a smaller volume of NaOH than the feed solution from which it was extracted, more than three-fold enhancements in acetic acid concentration were achieved with respect to the feed solution. Overall acetic acid recovery efficiencies of 57 & PLUSMN;1% and 46 & PLUSMN;2% were obtained for geraniol and eucalyptol, respectively. Both solvents were successfully used for multiple extraction/back-extraction cycles, with geraniol giving a stable concentration of back-extracted acetic acid over the course of ten cycles.

Conjugated polymers are increasingly exploited for biomedical applications. In this work, we explored the optical characteristics of conjugated polymers of variable chemical structures at multiple levels relevant to biological interfacing, from fluorescence yield to their influence on cellular membrane potential. We systematically compared the performance of conjugated polymer as cast thin films and as nanoparticles stabilized with amphiphilic polyethylene glycol-poly lactic acid-co-glycolic acid (PEG-PLGA). We assessed in both the dark and under illumination the stability of key optoelectronic properties in various environments, including air and biologically relevant physiological saline solutions. We found that photoreduction of oxygen correlates with nanoparticle and film degradation in physiologically relevant media. Using patch-clamp recordings in cell lines and primary neurons, we identified two broad classes of membrane potential response, which correspond to photosensitizer- and photothermal-mediated effects. Last, we introduced a metric named OED50 (optical energy for 50% depolarization), which conveys the phototoxic potency of a given agent and thereby its operational photo-safety profile.

Metallic nanogaps are fundamental components of nanoscale photonic and electronic devices. However, the lack of reproducible, high-yield fabrication methods with nanometric control over the gap-size is hindering practical applications. In this presentation, I will describe a novel technique called adhesion lithography that permits the rapid fabrication of nanogap electrodes, with electrode spacings as low as 3 nm. The procedure—which can be carried out at room temperature under ambient conditions, using simple equipment and only a few processing steps—provides a rapid and well-controlled route to a wide range of nanogap devices. I will describe how the basic technique may be combined with other lithographic methods to create large-area (> 1 cm2) arrays that contain hundreds of millions of size-controlled metallic nanogaps, and will give selected examples of how the resulting arrays may be applied in the fields of molecular electronics, plasmonics and biosensing.

Conjugated polymers display useful thermo-optical properties of high relevance to biomedical applications, which are not only dependent on their intrinsic chemical composition but also related to their physical conformation and manufacturing protocol. In this work, we report that the thermo-optical properties of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b ']sulated within poly(ethylene glycol) methyl ether-block-poly(lactide-co-glycolide) (PEG-PLGA) can be tuned by production conditions, generating conjugated polymer nanoparticles (CPNs) with customized applications. Thermal lens spectroscopy (TLS) was used to characterize the CPN light-to-heat conversion efficiency as it provides an absolute measurement of heat generation. Although preparation by traditional bulk production led to a high product yield, the CPNs were characterized by similar sizes and thermo-optical properties, irrespective of the molecular weight of amphiphilic PEG-PLGA. In contrast, a microfluidics production method generated CPNs with variable product yields and sizes and thermo-optical properties that are affected by both the molecular weight of PEG-PLGA and the production settings. Given the growing interest in biomedical applications of CPNs, our work provides useful results on microfluidic production of CPNs and of TLS for the screening of candidates with desirable characteristics.

Squeezing light into nanometer-sized metallic nanogaps can generate extremely high near-field intensities, resulting in dramatically enhanced absorption, emission, and Raman scattering of target molecules embedded within the gaps. However, the scarcity of low-cost, high-throughput, and reproducible nanogap fabrication methods offering precise control over the gap size is a continuing obstacle to practical applications. Using a combination of molecular self-assembly, colloidal nanosphere lithography, and physical peeling, we report here a high-throughput method for fabricating large-area arrays of triangular nanogaps that allow the gap width to be tuned from ∼10 to ∼3 nm. The nanogap arrays function as high-performance substrates for surface-enhanced Raman spectroscopy (SERS), with measured enhancement factors as high as 108 relative to a thin gold film. Using the nanogap arrays, methylene blue dye molecules can be detected at concentrations as low as 1 pM, while adenine biomolecules can be detected down to 100 pM. We further show that it is possible to achieve sensitive SERS detection on binary-metal nanogap arrays containing gold and platinum, potentially extending SERS detection to the investigation of reactive species at platinum-based catalytic and electrochemical surfaces.

Electric fields arising from the distribution of charge in metal halide perovskite solar cells are critical for understanding the many weird and wonderful optoelectronic properties displayed by these devices. Mobile ionic defects are thought to accumulate at interfaces to screen electric fields within the bulk of the perovskite semiconductor on application of external bias, but tools are needed to directly probe the dynamics of the electric field in this process. Here we show that electroabsorption measurements allow the electric field within the active layer to be tracked as a function of frequency or time. The magnitude of the electroabsorption signal, corresponding to the strength of the electric field in the perovskite layer, falls off for externally applied low frequency voltages or at long times following voltage steps. Our observations are consistent with drift-diffusion simulations, impedance spectroscopy, and transient photocurrent measurements. They indicate charge screening/redistribution on time-scales ranging from 10 ms to 100 s depending on the device interlayer material, perovskite composition, dominant charged defect, and illumination conditions. The method can be performed on typical solar cell structures and has potential to become a routine characterization tool for optimizing hybrid perovskite devices.

The Cover Feature shows the build instructions for the Open Polarimeter (“Opol”) – a high-resolution chemical polarimeter formed from a small number of inexpensive optomechanical parts. The complete instrument can be assembled from scratch in just two days for less than US$250, using only a 3D-printer and a benchtop milling machine. Despite its low cost, Opol achieves a high accuracy of a few millidegrees, comparable to far costlier commercial instruments. More information can be found in the Full Paper by John C. de Mello et al.

Metallic nanogaps with metal-metal separations of less than 10 nm have many applications in nanoscale photonics and electronics. However, their fabrication remains a considerable challenge, especially for applications that require patterning of nanoscale features over macroscopic length-scales. Here, some of the most promising techniques for nanogap fabrication are evaluated, covering established technologies such as photolithography, electron-beam lithography (EBL), and focused ion beam (FIB) milling, plus a number of newer methods that use novel electrochemical and mechanical means to effect the patterning. The physical principles behind each method are reviewed and their strengths and limitations for nanogap patterning in terms of resolution, fidelity, speed, ease of implementation, versatility, and scalability to large substrate sizes are discussed.

The Open Polarimeter (“Opol”) is a phase-based, high-resolution laser polarimeter formed from a small number of inexpensive optomechanical parts. The complete instrument can be assembled from scratch in two days for less than US$250, using only a 3D-printer and a benchtop milling machine. However despite its low cost Opol achieves a high accuracy of a few millidegrees, comparable to far costlier commercial instruments. It is released here as open hardware, with technical diagrams, a full parts list, and source-code for its firmware included as Supporting Information. Beyond polarimetry, Opol’s easy-to-build and versatile optical mounting system is likely to prove useful for a wide variety of optical systems.

We report a sensitive, fixed-wavelength, lock-in-based optical detector built from a light-emitting diode, two colour filters, a photodetector, a small number of discrete analogue components, and a low-cost microcontroller development board. We describe the construction, operating principle, use and performance of the optical detector, which may be used for both absorption- and fluorescence- measurements in either a 10-mm pathlength cuvette or a low-volume (< 100 μl) flow-cell. For illustrative purposes the detector is applied here to a cholesterol assay based on the enzyme-mediated conversion of (non-emissive) Amplex Red into the fluorescent dye resorufin, providing a detection limit of ~200 nM – some four orders of magnitude lower than the typical concentration of cholesterol in human serum. (The resorufin molecule itself is detectable down to concentrations of ~20 nM). The system may be readily adapted to other biomolecules through a simple change of enzyme.

The Front Cover shows a simple flow-based reactor for extracting copper ions from an impure feed solution. The extraction occurs in two stages. In the first stage, the feed solution (blue) is mixed with an immiscible organic solution (orange) that contains copper-selective extractant molecules. The copper ions rapidly transfer to the organic phase, which is then ‘tapped-off’ using a liquid-liquid separator. In the second stage, the organic solution is mixed with an acid solution (green), which strips the copper ions from the organic solvent. The two phases are separated using a second liquid/liquid separator. The copper solution passes straight through, while the organic solution is tapped-off and recycled through the reactor to carry out further extractions. More information can be found in the Full Paper by Andrew J. Harvie et al.

Invited for this month's cover is the group of John C. de Mello at the Norwegian University of Science and Technology in Trondheim (Norway). The cover picture shows a simple flow-based reactor for extracting copper ions from an impure feed solution. The extraction occurs in two stages. In the first stage, the feed solution (blue) is mixed with an immiscible organic solution (orange) that contains copper-selective extractant molecules. The copper ions rapidly transfer to the organic phase, which is then ‘tapped-off’ using a liquid-liquid separator. In the second stage, the organic solution is mixed with an acid solution (green), which strips the copper ions from the organic solvent. The two phases are separated using a second liquid/liquid separator. The copper solution passes straight through, while the organic solution is tapped-off and recycled through the reactor to carry out further extractions.  Read the full text of their Full Paper at 10.1002/cmtd.202100054 .

Metallic nanogaps are fundamental components of nanoscale photonic and electronic devices. However, the lack of reproducible high-yield fabrication methods with nanometric control over the gap-size has hindered practical applications. Here, we report a patterning technique based on molecular self-assembly and physical peeling that allows the gap-width to be tuned over the range 3 – 30 nm and enables the fabrication of massively parallel nanogap arrays containing hundreds of millions of ring-shaped nanogaps (RSNs). The method is used here to prepare molecular diodes across sub-3-nm metallic nanogaps and to fabricate visible-light-active plasmonic substrates based on large-area, gold-based RSN arrays. The substrates are applicable to a broad range of optical applications, and are used here as substrates for surface-enhanced Raman spectroscopy (SERS), providing high enhancement factors of up to 3e8 relative to similar, gap-free thin gold films.

Metal halide perovskites (MHPs) have excellent optoelectronic and photovoltaic applications because of their cost-effectiveness, tunable emission, high photoluminescence quantum yields, and excellent charge carrier properties. However, the potential applications of the entire MHP family are facing a major challenge arising from its weak resistance to moisture, polar solvents, temperature, and light exposure. A viable strategy to enhance the stability of MHPs could lie in their incorporation into a porous template. Metal-organic frameworks (MOFs) have outstanding properties, with a unique network of ordered/functional pores, which render them promising for functioning as such a template, accommodating a wide range of MHPs to the nanosized region, alongside minimizing particle aggregation and enhancing the stability of the entrapped species. This review highlights recent advances in design strategies, synthesis, characterization, and properties of various hybrids of MOFs with MHPs. Particular attention is paid to a critical review of the emergence of MHP@MOF for comprehensive studies of next-generation materials for various technological applications including sensors, photocatalysis, encryption/decryption, light-emitting diodes, and solar cells. Finally, by summarizing the state-of-the-art, some promising future applications of reported hybrids are proposed. Considering the inherent correlation and synergic functionalities of MHPs and MOFs, further advancement; new functional materials; and applications can be achieved through designing MHP@MOF hybrids.