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The rapid, point-of-care detection of copper in plasma can greatly aid in a large number of diseases where copper has been implicated to be an important factor, such as cancer, Alzheimer's and Diabetes mellitus. Localized surface plasmon resonance (LSPR) technologies show promise in the inexpensive detection of copper, whereas previous platforms are plagued with selectivity and sensitivity issues. Herein, we have created a sensitive and selective on-chip copper sensor which can produce a colorimetric reading in 60 minutes. The selectivity of the assay is based on 'Click' chemistry and is shown to have little interference with other metal ions present in plasma. The sensitivity of the assay is generated from the coupling of the molecular resonance of a dye and the LSPR of the gold nanoparticles. The assay is capable of measuring copper concentrations in human plasma as low as 4 µM and the linear range of sensitivity, 4 to 20 µM, is in the physiologically relevant range. This robust, colorimetric assay should prove useful in a point-of-care setting.

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A common capping agent for gold nanorods, Cetyl trimethylammonium bromide (CTAB), is particularly problematic for biological studies because of its cytotoxicity. Several procedures have been developed to remove the CTAB from the surface of the gold nanorods, but most are lengthy, involving many steps, and use expensive reagents. Here, we present a simple, one-pot method for the complete removal of CTAB from the surface of gold nanorods, so that particles can be more effectively utilized in biological in vivo studies. The procedure involves first adding sodium borohydride to remove the CTAB, quickly followed by a replacement ligand, such as mercaptoundecanoic acid (MUA). Both the CTAB removal and MUA replacement were monitored by FTIR, surface enhanced Raman spectroscopy (SERS) and X-Ray Photoelectron Spectroscopy (XPS) and compared to commercially available citrate-capped gold nanorods. The procedure presented herein is shown to be as effective at removing CTAB and replacing it with MUA as commercially available gold nanorod samples.

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Although great strides have been made in recent years toward making highly enhancing surface-enhanced Raman spectroscopy (SERS) substrates, the biological compatibility of such substrates remains a crucial problem. To address this issue, liposome-based SERS substrates have been constructed in which the biological probe molecule is encapsulated inside the aqueous liposome compartment, and metallic elements are assembled using the liposome as a scaffold. Therefore, the probe molecule is not in contact with the metallic surfaces. Herein we report our initial characterization of these novel nanoparticle-on-mirror substrates, both experimentally and theoretically, using finite-difference time-domain calculations. The substrates are shown to be structurally stable to laser irradiation, the liposome compartment does not rise above 45 °C, and they exhibit an analytical enhancement factor of 8 × 106 for crystal violet encapsulated in 38 liposomes sandwiched between a 40 nm planar gold mirror and 80 nm gold colloid.

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Integration of noble metal nanoparticles with proteins offers promising potential to create a wide variety of biosensors that possess both improved selectivity and versatility. The multitude of functionalities that proteins offer coupled with the unique optical properties of noble metal nanoparticles can allow for the realization of simple, colorimetric sensors for a significantly larger range of targets. Herein, we integrate the structural protein collagen with 10 nm gold nanoparticles to develop a protein-nanoparticle conjugate which possess the functionality of the protein with the desired colorimetric properties of the nanoparticles. Applying the many interactions that collagen undergoes in the extracellular matrix, we are able to selectively detect both glucose and heparin with the same collagen-nanoparticle conjugate. Glucose is directly detected through the cross-linking of the collagen fibrils, which brings the attached nanoparticles into closer proximity, leading to a red-shift in the LSPR frequency. Conversely, heparin is detected through a competition assay in which heparin-gold nanoparticles are added to solution and compete with heparin in the solution for the binding sites on the collagen fibrils. The collagen-nanoparticle conjugates are shown to detect both glucose and heparin in the physiological range. Lastly, glucose is selectively detected in 50% mouse serum with the collagen-nanoparticle devices possessing a linear range of 3-25 mM, which is also within the physiologically relevant range.

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Lipid membranes and membrane proteins are important biosensing targets, motivating the development of label-free methods with improved sensitivity. Silica-coated metal nanoparticles allow these systems to be combined with supported lipid bilayers for sensing membrane proteins through localized surface plasmon resonance (LSPR). However, the small sensing volume of LSPR makes the thickness of the silica layer critical for performance. Here, we develop a simple, inexpensive, and rapid sol-gel method for preparing thin conformal, continuous silica films and demonstrate its applicability using gold nanodisk arrays with LSPRs in the near-infrared range. Silica layers as thin as ∼5 nm are observed using cross-sectional scanning transmission electron microscopy. The loss in sensitivity due to the thin silica coating was found to be only 16%, and the biosensing capabilities of the substrates were assessed through the binding of cholera toxin B to GM1 lipids. This sensor platform should prove useful in the rapid, multiplexed detection and screening of membrane-associated biological targets.

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Localized surface plasmon resonance (LSPR) has emerged as a leader among label-free biosensing techniques in that it offers sensitive, robust, and facile detection. Traditional LSPR-based biosensing utilizes the sensitivity of the plasmon frequency to changes in local index of refraction at the nanoparticle surface. Although surface plasmon resonance technologies are now widely used to measure biomolecular interactions, several challenges remain. In this article, we have categorized these challenges into four categories: improving sensitivity and limit of detection, selectivity in complex biological solutions, sensitive detection of membrane-associated species, and the adaptation of sensing elements for point-of-care diagnostic devices. The first section of this article will involve a conceptual discussion of surface plasmon resonance and the factors affecting changes in optical signal detected. The following sections will discuss applications of LSPR biosensing with an emphasis on recent advances and approaches to overcome the four limitations mentioned above. First, improvements in limit of detection through various amplification strategies will be highlighted. The second section will involve advances to improve selectivity in complex media through self-assembled monolayers, "plasmon ruler" devices involving plasmonic coupling, and shape complementarity on the nanoparticle surface. The following section will describe various LSPR platforms designed for the sensitive detection of membrane-associated species. Finally, recent advances towards multiplexed and microfluidic LSPR-based devices for inexpensive, rapid, point-of-care diagnostics will be discussed.

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Development of improved glucose detection has vast significance in both clinical and point of care settings. Herein, we present a novel, label-free, enzyme-free, colorimetric method of glucose detection that relies on the reduction of a gold salt precursor facilitated by physiological concentrations of glucose (1.25-50 mM). The concentration of glucose present during the reduction process results in nanoparticles of different size, which in turn change the color of solution. Through transmission electron microscopy (TEM), it was found that the nanoparticle size decreases as the glucose concentration increases. Kinetic characterization of nanoparticle formation shows rate constants change 5-8 orders of magnitude when comparing normal versus diabetic glucose concentrations. Assay versatility was also investigated through incorporation onto solid substrates as well as the addition of a filtering step, which produced relatively clear samples below the diabetic cut-off (10 mM glucose) and colored samples above. The colorimetric sensor was then found to also show similar color changes with glucose solutions containing biological interfering agents as well as samples with 20% serum. Last, the sensor was tested in solution containing 100% mouse serum and 100% bovine urine spiked with varying glucose concentrations, which resulted in smaller nanoparticle formation whose intensities were dependent on glucose concentration. The resulting color changes observed for this sensor in urine samples are directly compared with Benedict's reagent and are shown to be significantly more sensitive to lower concentrations of glucose in the diabetic relevant range.

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The solution nuclear magnetic resonance (NMR) structures and backbone (15)N dynamics of the specialized acyl carrier protein (ACP), RpAcpXL, from Rhodopseudomonas palustris, in both the apo form and holo form modified by covalent attachment of 4'-phosphopantetheine at S37, are virtually identical, monomeric, and correspond to the closed conformation. The structures have an extra α-helix compared to the archetypical ACP from Escherichia coli, which has four helices, resulting in a larger opening to the hydrophobic cavity. Chemical shift differences between apo- and holo-RpAcpXL indicated some differences in the hinge region between α2 and α3 and in the hydrophobic cavity environment, but corresponding changes in nuclear Overhauser effect cross-peak patterns were not detected. In contrast to the NMR structures, apo-RpAcpXL was observed in an open conformation in crystals that diffracted to 2.0 Å resolution, which resulted from movement of α3. On the basis of the crystal structure, the predicted biological assembly is a homodimer. Although the possible biological significance of dimerization is unknown, there is potential that the resulting large shared hydrophobic cavity could accommodate the very long-chain fatty acid (28-30 carbons) that this specialized ACP is known to synthesize and transfer to lipid A. These structures are the first representatives of the AcpXL family and the first to indicate that dimerization may be important for the function of these specialized ACPs.

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