Nanoplasmonic Sensing Research Articles

Nanoplasmonic Sensing at the Carbon-Bio Interface: Study of Protein Adsorption at Graphitic and Hydrogenated Carbon Surfaces.

Various forms of carbon are known to perform well as biomaterials in a variety of applications and an improved understanding of their interactions with biomolecules, cells, and tissues is of interest for improving and tailoring their performance. Nanoplasmonic sensing (NPS) has emerged as a powerful technique for studying the thermodynamics and kinetics of interfacial reactions. In this work, the in situ adsorption of two proteins, bovine serum albumin and fibrinogen, were studied at carbon surfaces with differing chemical and optical properties using nanoplasmonic sensors. The carbon material was deposited as a thin film onto NPS surfaces consisting of 100 nm Au nanodisks with a localized plasmon absorption peak in the visible region. Carbon films were fully characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Two types of material were investigated: amorphous carbon (a-C), with high graphitic content and high optical absorptivity, and hydrogenated amorphous carbon (a-C:H), with low graphitic content and high optical transparency. The optical response of the Au/carbon NPS elements was modeled using the finite difference time domain (FDTD) method, yielding simulated analytical sensitivities that compare well with those observed experimentally at the two carbon surfaces. Protein adsorption was investigated on a-C and a-C:H, and the protein layer thicknesses were obtained from FDTD simulations of the expected response, yielding values in the 1.8-3.3 nm range. A comparison of the results at a-C and a-C:H indicates that in both cases fibrinogen layers are thicker than those formed by albumin by up to 80%.

Unraveling Interactions between Ionic Liquids and Phospholipid Vesicles Using Nanoplasmonic Sensing

Owing to their unique properties and unlimited structural combinations, the ubiquitous use of ionic liquids (ILs) is steadily increasing. The objective of the present work is to shed light onto the effects of amidinium- and phosphonium-based ILs on phospholipid vesicles using a nanoplasmonic sensing measurement technique. A new and relatively simple method was developed for the immobilization of large unilamellar vesicles on two different hydrophilic surfaces composed of titanium dioxide and silicon nitride nanolayers. Among the pretreatment conditions studied, vesicle attachment on both substrate materials was achieved with HEPES buffer in the presence of sodium hydroxide and calcium chloride. To get an understanding of how ILs interact with intact vesicles or with supported lipid bilayers, the ILs 1,5-diazabicyclo(4.3.0)non-5-enium acetate ([DBNH][OAc]), tributyl(tetradecyl)phosphonium acetate ([P14444][OAc]), and tributylmethylphosphonium acetate ([P4441][OAc]) were introduced into the biomimetic system, and the characteristics of their interactions with the immobilized vesicles were determined. Depending on the IL, in situ real-time IL binding and/or phospholipid removal processes were observed. Although [DBNH][OAc] did not have any significant effect on the phospholipid vesicles, the strongest and the most significant effect was observed with [P14444][OAc]. The latter caused clear changes in the phospholipid bilayer: the ILs interacted with the bilayers, resulting in deformation of the vesicles most probably due to the formation of vesicle-IL aggregates. Only a mild effect was observed when [P4441][OAc], at a very high concentration, was exposed to the intact vesicles. In general, these results led to new insights into the effects of ILs on phospholipid vesicles, which are of great importance to the overall understanding of the harmfulness of ILs on biomembranes and biomimicking systems. In addition, the present work highlights the pivotal role of this highly surface-sensitive indirect biosensing technique in scrutinizing and dissecting the integrity and architecture of phospholipid vesicles in the nanoscale range.

Nanoplasmonic Biological Sensing and Imaging

Nanoplasmonic Biological Sensing and Imaging

(Corrosion Division Morris Cohen Graduate Student Award) In-situ Monitoring of Ultra-slow Oxide Growth on Copper

The atmospheric corrosion of metals and alloys is a very complex process, which involves a wide range of chemical, electrochemical, and physical processes occurring in gas, liquid, and solid phases, as well as at the interfaces between them. Essentially all metals and alloys undergo corrosion in indoor or outdoor atmospheric conditions due to the presence of gaseous corrosion stimulators. However, the rate of the atmospheric corrosion might vary significantly depending on the nature of the metals or alloys, the concentration of corrosive species, as well as the exposure conditions. In some applications such as electrical contacts in microelectronics, military equipment or nuclear waste containers, even ultra-slow oxidation/corrosion may result in catastrophic consequences. In order to study such slow oxidation/corrosion processes, techniques involving ultra-high vacuum (UHV) are commonly used. Despite a very high surface sensitivity of these techniques they suffer from the drawback that the experiments are not performed under normal ambient conditions, but rather under vacuum. This has the consequence that the water adlayer which always exists on metal surfaces in atmospheric environments is removed under UHV conditions. Furthermore, in the case of ex-situ UHV studies, the structure of the oxidation/corrosion products might undergo vacuum induced changes. Hence, in-situ and highly surface sensitive techniques capable of working under atmospheric pressures are desired to monitor very slow oxidation and corrosion processes on metals and alloys at a molecular level. In the present study, several analytical techniques have been used to obtain a picture as complete as possible of an atmospheric corrosion process, focusing on both the formation of corrosion products and the evolution of the protective layer. Copper surfaces covered with protective organic monolayers have been exposed to various atmospheric environments with the aim of simulating typical indoor atmospheric corrosion processes. Infrared reflection/absorption spectroscopy (IRAS) was used to determine the nature of the corrosion products, quartz crystal microbalance (QCM) to measure the mass of them, and nano plasmonic sensing (NPS) to determine the corrosion rate as well as to evaluate how homogeneous the formation of the corrosion products was. In addition, an inherently surface sensitive technique, vibrational sum frequency spectroscopy (VSFS) was used to study induced changes in the molecular structure of the protective monolayers. VSFS studies of octadecanethiol (ODT) covered copper surfaces exposed to dry air allowed the detection of the formation of thin copper oxide layers and the observation of accompanying changes in the molecular structure of the hydrocarbon chains of ODT. VSFS was combined with QCM to quantify the amount of copper oxide formed on the surface. Since oxidation and corrosion processes often occur heterogeneously, the VSFS-QCM setup was integrated with NPS, which enabled us to distinguish homogeneous and heterogeneous corrosion under in-situ conditions with a time resolution on the order of seconds. This unique combination of three in-situ techniques facilitates the study of different corrosion processes where interfacial changes in electronic and optical properties as well as changes in mass occur. The sensitivity of the techniques under current exposure conditions corresponds to 5% of an oxide monolayer for VSFS, 2.5% for QCM, and 1.2% for NPS. As an extension of the above mentioned studies a combination of VSFS and IRAS was used to compare the corrosion protection efficiency of alkanethiol SAMs with different chain lengths and to compare SAMs with –S or –Se as their anchoring group when the samples were exposed to humidified air (HA) containing formic acid (FA). It was found that the corrosion inhibition efficiency of alkanethiol SAMs on copper was enhanced with an increased chain length. Furthermore, alkanethiol SAMs feature “selective hindrance” properties, i.e. they selectively allow the penetration of formic acid and oxygen and hinder the penetration of water resulting in the formation of copper formate and copper hydroxide. Interestingly, no formation of copper oxide was observed, which was in contrast to unprotected copper. VSFS revealed that SAMs of alkanethiols get more disordered over the course of sample exposure to HA+FA but remain attached to the substrate. Alkaneselenol SAMs initially behave similarly to their alkanethiol counterparts. However, after prolonged exposure to HA+FA they are partially removed from the surface, resulting in an accelerated corrosion of the copper substrate. Eventually, the corrosion products formed on alkaneselenol SAM covered samples consists of copper formate, copper hydroxide, and copper oxide. Figure 1

Core–Shell Nanoplasmonic Sensing for Characterization of Biocorona Formation and Nanoparticle Surface Interactions

Surface properties of nanoparticles imposed by particle size, shape, and surface chemistry are key features that largely determine their environmental fate and effects on biological systems. Consequently, development of analytical tools to characterize surface properties of nanomaterials and their relation to toxicological properties must occur in parallel with applications. As a contribution to this quest, we present a nanoplasmonic sensing strategy that enables systematic in situ characterization of molecule–nanoparticle interactions under well-controlled conditions, in terms of both nanoparticle size and surface chemistry, with particular focus on the importance of surface faceting in crystalline nanoparticles. We assess the performance of our sensing strategy by presenting two case studies. (i) The first is protein corona formation on faceted Au core–SiO2 shell nanoparticles of different sizes, and thus different surface facet-to-edge ratios. Based on 2D and 3D models of the investigated structures, we find that for small particles the curved regions between adjacent facets dominate the response of the corona formation process, whereas the facets dominate the response in the large particle regime. (ii) The second is in situ functionalization of Au core–SiO2 shell nanoparticle surfaces, and analysis of the subsequent protein repellent behavior. Due to the versatility of the presented sensing strategy in studies of nanoparticle surface properties, including in situ surface modifications, and their interactions with (bio)molecules during corona formation, we foresee it to become a valuable tool in the areas of nanomedicine and nanotoxicology.

Indirect Nanoplasmonic Sensing to Probe with a High Sensitivity the Interaction of Water Vapor with Soot Aerosols

We demonstrate in this work that the indirect nanoplasmonic sensing lets us follow the adsorption/desorption of water molecules on soot particles that are a major contributor of the global warming. Increasing the relative humidity of the surrounding medium we measure a shift in wavelength of the localized surface plasmon resonance response of gold nanodisks on which soot particles are deposited. We show a singular and reversible blue shift with hydrophilic aircraft soot particles interpreted from a basic model as a reversible morphological change of the soot aggregates. This new method is highly sensitive and interesting to follow the change of optical properties of aerosols during their aging in the atmosphere, where they can adsorb and react with different gas molecules.

Reduction of Cu-Promoted Fe Model Catalysts Studied by In Situ Indirect Nanoplasmonic Sensing and X-ray Photoelectron Spectroscopy

The reduction of Cu-promoted Fe model catalysts was investigated using X-ray photoelectron spectroscopy (XPS) and indirect nanoplasmonic sensing (INPS). The catalysts were prepared by evaporation of 1 nm thick Fe particle films onto quartz wafers, followed by deposition of 0, 0.2, or 0.02 nm of Cu (i.e., 0, 2, or 19 wt %) as a promoter. For the XPS measurements, a reaction cell with in vacuo transfer to the measurement chamber was used. The catalysts were first oxidized at 400 degrees C in Ar/O-2, achieving fully oxidized Fe2O3 and CuO. Subsequently, the samples were heated to temperatures between 100 and 400 degrees C in pure H-2, and the resulting change in oxidation state was measured. Fe2O3 was found to be reduced to Fe3O4 at 225 degrees C and to a mixed state of FeO and metallic Fe at 275 degrees C. The corresponding temperatures for Cu-promoted Fe catalysts were 100 degrees C lower. In the absence of FeOx, Cu was reduced to metallic Cu, via Cu2O, at temperatures between 125 and 175 degrees C. In addition to the XPS measurements, INPS was used to obtain more detailed insight into the reduction process, both in pure H-2 and in wet H-2, containing 0.8 vol % H2O. These in situ experiments show that the presence of H2O increases the reduction temperature by 36 degrees C or more, depending on the amount of Cu promoter used, where the catalyst with the industrially relevant 2 wt % promoter material exhibits the smallest increase. The INPS measurements also demonstrate that increasing the amount of Cu promoter decreases the Fe2O3 reduction temperature, in both dry and wet H-2. Together, XPS and INPS offer a powerful combination for monitoring the oxidation state of flat model catalysts during pretreatments, an approach that can equally well be used during catalytic reaction conditions.

Integrated Nanoplasmonic Sensing for Cellular Functional Immunoanalysis Using Human Blood

Localized surface plasmon resonance (LSPR) nanoplasmonic effects allow for label-free, real-time detection of biomolecule binding events on a nanostructured metallic surface with simple optics and sensing tunability. Despite numerous reports on LSPR bionanosensing in the past, no study thus far has applied the technique for a cytokine secretion assay using clinically relevant immune cells from human blood. Cytokine secretion assays, a technique to quantify intercellular-signaling proteins secreted by blood immune cells, allow determination of the functional response of the donor’s immune cells, thus providing valuable information about the immune status of the donor. However, implementation of LSPR bionanosensing in cellular functional immunoanalysis based on a cytokine secretion assay poses major challenges primarily owing to its limited sensitivity and a lack of sufficient sample handling capability. In this paper, we have developed a label-free LSPR biosensing technique to detect cell-secreted tumor necrosis factor (TNF)-α cytokines in clinical blood samples. Our approach integrates LSPR bionanosensors in an optofluidic platform that permits trapping and stimulation of target immune cells in a microfluidic chamber with optical access for subsequent cytokine detection. The on-chip spatial confinement of the cells is the key to rapidly increasing a cytokine concentration high enough for detection by the LSPR setup, thereby allowing the assay time and sample volume to be significantly reduced. We have successfully applied this approach first to THP-1 cells and then later to CD45 cells isolated directly from human blood. Our LSPR optofluidics device allows for detection of TNF-α secreted from cells as few as 1000, which translates into a nearly 100 times decrease in sample volume than conventional cytokine secretion assay techniques require. We achieved cellular functional immunoanalysis with a minimal blood sample volume (3 μL) and a total assay time 3 times shorter than that of the conventional enzyme-linked immunosorbent assay (ELISA).

Combined in Situ Quartz Crystal Microbalance with Dissipation Monitoring, Indirect Nanoplasmonic Sensing, and Vibrational Sum Frequency Spectroscopic Monitoring of Alkanethiol-Protected Copper Corrosion

In this study, we have applied three techniques to simultaneously and in situ study the initial stage of corrosion of copper protected by a self-assembled monolayer of octadecanethiol (ODT). We combined quartz crystal microbalance with dissipation monitoring (QCM-D), indirect nanoplasmonic sensing (INPS), and vibrational sum frequency spectroscopy (VSFS) and obtained complementary information about mass uptake and optical and spectroscopic changes taking place during the initial corrosion phase. All three techniques are very sensitive to the formation of a corrosion film (thickness in the range 0-0.41 nm) under mildly corrosive conditions (dry air, <0.5% relative humidity). The three techniques yield information about the viscoelasticity of the corrosion film (QCM-D), the homogeneity of the corrosion reaction on the surface (INPS), and the stability of the ODT protection layer (VSFS). Furthermore, by also studying the corrosion process in humid air (ca. 70% relative humidity), we illustrate how the combination of these techniques can be used to differentiate between simultaneously occurring processes, such as water adsorption and corrosion product formation.

Diffusion and adsorption of dye molecules in mesoporous TiO2 photoelectrodes studied by indirect nanoplasmonic sensing

In this study, we used Hidden Interface-Indirect Nanoplasmonic Sensing (HI-INPS) for real time monitoring of dye impregnation (adsorption–diffusion process) of mesoporous TiO2 electrodes of the kind used in dye-sensitized solar cells. We measured the dye percolation time (i.e. the time to diffuse to the bottom of a TiO2 photoelectrode film) for dye Z907 in a 1 : 1 volume mixture of acetonitrile and tert-butanol for different dye concentrations and for different thicknesses of the TiO2 film, while the total amount of adsorbed dye was simultaneously measured by optical absorption spectroscopy. The experimental data for the impregnation process were analyzed by employing a diffusion-front model, combining diffusion and Langmuir type adsorption, which allows extraction of the effective diffusion coefficient for the system. The latter value is about 15 μm2 s−1 for the combined adsorption–diffusion movement of dye molecules through the TiO2 structure, which is an order of magnitude or more smaller than that for “free” diffusion of dye molecules in bulk solvents.

Time-Resolved Indirect Nanoplasmonic Sensing Spectroscopy of Dye Molecule Interactions with Dense and Mesoporous TiO2 Films

Indirect nanoplasmonic sensing (INPS) is an experimental platform exploiting localized surface plasmon resonance (LSPR) detection of processes in nanomaterials, molecular assemblies, and films at the nanoscale. Here we have for the first time applied INPS to study dye molecule adsorption/impregnation of two types of TiO(2) materials: thick (10 μm) mesoporous films of the kind used as photoanode in dye-sensitized solar cells (DSCs), with particle/pore size in the range of 20 nm, and thin (12-70 nm), dense, and flat films. For the thick-film experiments plasmonic Au nanoparticles were placed at the hidden, internal interface between the sensor surface and the mesoporous TiO(2). This approach provides a unique opportunity to selectively follow dye adsorption locally in the hidden interface region inside the material and inspires a generic and new type of nanoplasmonic hidden interface spectroscopy. The specific DSC measurement revealed a time constant of thousands of seconds before the dye impregnation front (the diffusion front) reaches the hidden interface. In contrast, dye adsorption on the dense, thin TiO(2) films exhibited much faster, Langmuir-like monolayer formation kinetics with saturation on a time scale of order 100 s. This new type of INPS measurement provides a powerful tool to measure and optimize dye impregnation kinetics of DSCs and, from a more general point of view, offers a generic experimental platform to measure adsorption/desorption and diffusion phenomena in solid and mesoporous systems and at internal hidden interfaces.

Nanoplasmonic Sensing and QCM-D as Ultrasensitive Complementary Techniques for Corrosion Studies

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Nanoplasmonic Sensing for (Nano)Materials Science - Complementation with QCM-D and Quantification

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Identification of the Optimal Spectral Region for Plasmonic and Nanoplasmonic Sensing

We present a theoretical and experimental study involving the sensing characteristics of wavelength-interrogated plasmonic sensors based on surface plasmon polaritons (SPP) in planar gold films and on localized surface plasmon resonances (LSPR) of single gold nanorods. The tunability of both sensing platforms allowed us to analyze their bulk and surface sensing characteristics as a function of the plasmon resonance position. We demonstrate that a general figure of merit (FOM), which is equivalent in wavelength and energy scales, can be employed to mutually compare both sensing schemes. Most interestingly, this FOM has revealed a spectral region for which the surface sensitivity performance of both sensor types is optimized, which we attribute to the intrinsic dielectric properties of plasmonic materials. Additionally, in good agreement with theoretical predictions, we experimentally demonstrate that, although the SPP sensor offers a much better bulk sensitivity, the LSPR sensor shows an approximately 15% better performance for surface sensitivity measurements when its FOM is optimized. However, optimization of the substrate refractive index and the accessibility of the relevant molecules to the nanoparticles can lead to a total 3-fold improvement of the FOM in LSPR sensors.

Hydrogen Storage in Pd Nanodisks Characterized with a Novel Nanoplasmonic Sensing Scheme

A novel nanoplasmonic sensing scheme is introduced based on remote real-time detection of induced electronic and shape/structural changes in a metal nanoparticle during the metal-hydride formation process. The localized surface plasmon resonance (LSPR) of the nanoparticle is utilized as signal transducer for optical readout. As a model system, hydrogen storage through metal-hydride formation is studied in Pd nanodisks. The experimentally obtained plasmonic response to hydrogen uptake yields pressure-LSPR-response isotherms. These isotherms are found to obey Sievert's law in the low-pressure range and exhibit a characteristic "plateau" at 18 Torr upon hydrogen charging and 7.5 Torr upon hydrogen discharging. An additional experiment also clearly shows the typical temperature dependence of the plateau pressure. Conversion of the LSPR signal to absolute hydrogen concentration, based on a proposed linear dependence of the LSPR response to hydrogen uptake, results in p-C isotherms in excellent agreement with those in the literature. This puts forward that the LSPR response is an extremely sensitive, remote, and real-time probe for "bulk" changes in a metal nanoparticle and can readily be used to study processes such as metal-hydride formation for hydrogen storage applications, alloying on the nanoscale, thermal reshaping, and so forth.