Philip Howes

Colloidal nanoparticles as advanced biological sensors

Background Nanoparticle biosensors have the potential to enhance or supersede current analytical techniques, and their introduction could have a great impact in research and clinical practice. The unique optical properties of many nanomaterials make them ideal for biosensing. Colloidal fluorescent and plasmonic nanoparticles are particularly interesting, as they produce intense responses to incident light, and linking this response to the presence of a target analyte yields extremely sensitive detection in solution. Optimization of nanoparticle biosensors by considering the nanoparticle core, surface, and sensing properties, with an eye toward their application in research and as clinical tools. This translational process relies on the availability of high-quality nanoparticles with precisely engineered surfaces and biosensing mechanisms that allow detection with high sensitivity and specificity. Advances Much research has centered on fluorescent quantum dots (QDs) and plasmonic gold nanoparticles (AuNPs) and has expanded to include carbon dots, silicon dots, upconverting nanoparticles, alloyed plasmonic nanoparticles, and gold and silver nanoclusters, among a constantly growing repertoire. New tools for probing the nucleation and growth of nanoparticles, such as in situ synchrotron x-ray irradiation, liquid cell transmission electron microscopy, and computer simulation, are revealing ever more about fundamental processes—for example, the importance of particle coalescence and surface ligand conformation during particle growth. Precision engineering of particle surfaces is required to construct advanced nanoparticle biosensors, and this is being facilitated by a new generation of modular small-molecule and polymeric capping agents, the incorporation of new high-yield bio-orthogonal “click”-like functional groups, and advanced engineering of biomolecular sensing constructs (e.g., by recombinant protein engineering). Notable successes in the field include the precision synthesis of nanoparticles in microfluidic systems; ultrasensitive detection of cancer biomarkers in human serum with time-gated QD fluorescence; multiplexed intracellular sensing of mRNA using superquenching AuNPs; multiplexed detection of analytes with simple technologies such as smartphones; in vivo sensing of reactive oxygen species and methyl mercury; and the integration of nanoparticle biosensors with advanced DNA/RNA target amplification protocols. Outlook Many advanced applications are anticipated for nanoparticle biosensors, including as research analytical tools, intracellular sensors, and in vivo sensors for real-time detection and visualization of analytes and structures inside the body. There has been great success in synthesizing nanoparticles with excellent physical and chemical properties and in demonstrating their applications as biosensors. Looking toward high-impact applications, the main challenges are to develop techniques to synthesize reproducible high-quality nanoparticle biosensors on a mass scale and to maximize performance in physiological conditions. By drawing on many fields, including molecular biology, bioinformatics, computer modeling, bioengineering, and applied physics, we can compile the diverse toolset required to overcome these challenges and move toward high-impact applications of nanoparticle biosensors. Biological sensing using nanoparticles Colloidal fluorescent and plasmonic nanoparticles yield intense responses to incident light, making them useful as sensors or probes for sensitive detection in solution. Howes et al. review the potential uses of nanoparticle biosensors in research and diagnostics. A range of methods allow for the chemical modification of the particle surfaces so that they can be tuned for specific analytes and give optical signals for a range of biological conditions of interest. Signals can be detected in complex media or in vivo making the particles of interest for both laboratory research and in clinical settings. Science, this issue 10.1126/science.1247390 Colloidal nanoparticle biosensors have received intense scientific attention and offer promising applications in both research and medicine. We review the state of the art in nanoparticle development, surface chemistry, and biosensing mechanisms, discussing how a range of technologies are contributing toward commercial and clinical translation. Recent examples of success include the ultrasensitive detection of cancer biomarkers in human serum and in vivo sensing of methyl mercury. We identify five key materials challenges, including the development of robust mass-scale nanoparticle synthesis methods, and five broader challenges, including the use of simulations and bioinformatics-driven experimental approaches for predictive modeling of biosensor performance. The resultant generation of nanoparticle biosensors will form the basis of high-performance analytical assays, effective multiplexed intracellular sensors, and sophisticated in vivo probes.

Phospholipid Encapsulated Semiconducting Polymer Nanoparticles: Their Use in Cell Imaging and Protein Attachment

Semiconducting polymer nanospheres (SPNs) have been synthesized and encapsulated in phospholipid micelles by a solvent evaporation technique. Four different conjugated polymers were used, yielding aqueous dispersions of nanoparticles which emit across the visible spectrum. The synthesis was simple and easily reproducible, and the resultant nanoparticle solutions exhibited high colloidal stability. As these encapsulated SPNs do not contain any toxic materials and show favorable optical properties, they appear to be a promising imaging agent in biomedical and imaging applications. The SPNs were used in simple fluorescence imaging experiments and showed uptake in SH-SY5Y neuroblastoma and live HeLa cells. Carboxylic acid functionalized SPNs were also synthesized and conjugated to bovine serum albumin (BSA) by carbodiimide-mediated chemistry, a key step in the realization of targeted imaging using conjugated polymers.

Magnetic conjugated polymer nanoparticles as bimodal imaging agents.

Hybrid nanoparticles which incorporate multiple functionalities, such as fluorescence and magnetism, can exhibit enhanced efficiency and versatility by performing several tasks in parallel. In this study, magnetic-fluorescent semiconductor polymer nanospheres (MF-SPNs) have been synthesized by encapsulation of hydrophobic conjugated polymers and iron oxide nanoparticles in phospholipid micelles. Four fluorescent conjugated polymers were used, yielding aqueous dispersions of nanoparticles which emit across the visible spectrum. The MF-SPNs were shown to be magnetically responsive and simultaneously fluorescent. In MRI studies, they were seen to have a shortening effect on the transverse T(2)* relaxation time, which demonstrates their potential as an MR contrast agent. Finally, successful uptake of the MF-SPNs by SH-SY5Y neuroblastoma cells was demonstrated, and they were seen to behave as bright and stable fluorescent markers. There was no evidence of toxicity or adverse affect on cell growth.

A review: On the development of low melting temperature Pb-free solders

Pb-based solders have been the cornerstone technology of electronic interconnections for many decades. However, with legislation in the European Union and elsewhere having moved to restrict the use of Pb, it is imperative that new Pb-free solders are developed which can meet the long established benchmarks set by leaded solders and improve on the current generation of Pb free solders such as SAC105 and SAC305. Although this poses a great challenge to researchers around the world, significant progress is being made in developing new solder alloys with promising properties. In this review, we discuss fundamental research activity and its focus on the solidification and interfacial reactions of Sn-based solder systems. We first explain the reactions between common base materials, coatings, and metallisations, and then proceed to more complex systems with additional alloying elements. We also discuss the continued improvement of substrate resistance to attack from molten Sn which will help maintain the interface stability of interconnections. Finally, we discuss the various studies which have looked at employing nanoparticles as solder additives, and the future prospects of this field.

Luminescent quantum-dot-sized conjugated polymer nanoparticles—nanoparticle formation in a miniemulsion system

The synthesis of semiconducting polymer nanospheres (SPNs) by miniemulsion methods has been reported previously,1,2 however the resulting materials have typically been significantly larger than quantum dots, the nanomaterial of choice for cell imaging. In the current work, we aimed to obtain quantum-dot-sized poly(ethylene-glycol) (PEG) functionalised SPNs by optimising the miniemulsion method reported previously.1,3 We report that PEG functionalised SPNs with a narrow distribution of quantum dot sizes were successfully synthesised, with mean diameters ranging between 2 and 5 nm. PEG-dithiol was also used in one variant of the syntheses, allowing the purified sample to be examined by mass spectrometry (using sulfur as a reporter), confirming the inclusion of a significant number of PEG molecules in the nanoparticles. Also, the presence of PEG was found to significantly affect the yield of the reaction, suggesting it played a role in nanoparticle formation. The nanoparticles were characterized by TEM, absorption and emission spectroscopy, and were found to be stable in solution for months.

Simple conjugated polymer nanoparticles as biological labels

The use of nanoparticles in biology, especially in cellular imaging, is extremely promising and offers numerous advantages over existing organic dye systems. There are, however, constraints that need to be addressed before the use of such materials in mainstream clinical applications can be realized. One of the main concerns is the use of metal-containing particles that are potentially toxic or interfere with other diagnostic processes. Here, we present the use of simple conjugated polymer nanoparticles as alternative photostable cellular optical imaging agents.

Synthesis and shape control of mercury selenide (HgSe) quantum dots

Mercury selenide (HgSe) quantum dots have been synthesised and capped with the coordinating solvent trioctylphosphine oxide (TOPO), which mediated growth and stabilised the particles. By varying the precursor to surfactant ratio, differing particle morphologies were obtained.

Colloidal and optical stability of PEG-capped and phospholipid-encapsulated semiconducting polymer nanospheres in different aqueous media

Aqueous dispersions of fluorescent semiconducting polymer nanospheres (SPNs) have been synthesised by two methods; miniemulsion and micellar encapsulation. The colloidal and optical stability of SPNs synthesised by these two methods has been compared in order to assess the potential of these fluorescent nanoparticles for use in biological applications. The SPNs were dispersed in water, phosphate buffer solution (PBS) and bovine serum albumin (BSA). The optical stability was studied by absorption and emission spectroscopy, and the colloidal stability was studied by dynamic light scattering (DLS) over a one month period. The results indicate that the micelle-encapsulated SPNs exhibit favourable optical and colloidal stability, and seem promising for use in biological sciences.

Multi-Amplified Sensing of MicroRNA by a Small DNA Fragment-Driven Enzymatic Cascade Reaction

Combining technological developments such as nanomaterials, DNA nanotechnology, and functional enzymes has great potential to facilitate next generation high performance molecular diagnostic systems. In this work, we describe a microRNA (miRNA) detection assay that combines target recycling and isothermal amplification in an elegantly designed enzyme-mediated cascade reaction. Target recycling is driven by the action of duplex-specific nuclease (DSN), resulting in highly amplified translation of input miRNA to short output DNA fragments. These fragments act as highly specific initiators of rolling circle amplification (RCA), an isothermal reaction that outputs a large volume of polymeric DNAzymes per initiator, and finally a fluorogenic output signal. Based on careful electrophoretic analysis we observed that the DSN produces ca. 10 nt DNA fragments from DNA/miRNA duplexes, regardless of the length of DNA strands. Target recycling yielded ca. 5 orders of magnitude amplification through the DSN-assisted recycling system on magnetic particles, and the RCA yielded a further 2 orders of magnitude. The final assay exhibited a limit of detection of 1.8 fM of miRNA spiked into 20% human serum, and showed excellent selectivity for miR-21 versus single base-mismatched sequences and other cancer-related miRNAs. The developed assay was further employed to determine accurate amounts of miR-21 in total RNA samples extracted from human cancer cell lines and normal cells, confirming the applicability of the assay for direct and absolute quantification of mature specific miRNA in real biological samples.

The perception of materials through oral sensation.

This paper presents the results of a multimodal study of oral perception conducted with a set of material samples made from metals, polymers and woods, in which both the somatosensory and taste factors were examined. A multidimensional scaling analysis coupled with subjective attribute ratings was performed to assess these factors both qualitatively and quantitatively. The perceptual somatosensory factors of warmth, hardness and roughness dominated over the basic taste factors, and roughness was observed to be a less significant sensation compared to touch-only experiments. The perceptual somatosensory ratings were compared directly with physical property data in order to assess the correlation between the perceived properties and measured physical properties. In each case, a strong correlation was observed, suggesting that physical properties may be useful in industrial design for predicting oral perception.