Abdennour Abbas

New trends in instrumental design for surface plasmon resonance-based biosensors

Surface plasmon resonance (SPR)-based biosensing is one of the most advanced label free, real time detection technologies. Numerous research groups with divergent scientific backgrounds have investigated the application of SPR biosensors and studied the fundamental aspects of surface plasmon polaritons that led to new, related instrumentation. As a result, this field continues to be at the forefront of evolving sensing technology. This review emphasizes the new developments in the field of SPR-related instrumentation including optical platforms, chips design, nanoscale approach and new materials. The current tendencies in SPR-based biosensing are identified and the future direction of SPR biosensor technology is broadly discussed.

Plasmonic Planet–Satellite Analogues: Hierarchical Self-Assembly of Gold Nanostructures

In the past few years, a remarkable progress has been made in unveiling novel and unique optical properties of strongly coupled plasmonic nanostructures, known as plasmonic molecules. However, realization of such plasmonic molecules using nonlithographic approaches remains challenging largely due to the lack of facile and robust assembly methods. Previous attempts to achieve plasmonic nanoassemblies using molecular ligands were limited to dipolar assembly of nanostructures, which typically results in polydisperse linear and branched chains. Here, we demonstrate that core-satellite structures comprised of shape-controlled plasmonic nanostructures can be achieved through self-assembly using simple molecular cross-linkers. Prevention of self-conjugation and promotion of cross-conjugation among cores and satellites plays a key role in the formation of core-satellite heteroassemblies. The in-built electromagnetic hot-spots and Raman reporters of core-satellite structures make them excellent candidates for surface-enhanced Raman scattering probes.

Multifunctional analytical platform on a paper strip: separation, preconcentration, and subattomolar detection.

We report a plasmonic paper-based analytical platform with functional versatility and subattomolar (<10(-18) M) detection limit using surface-enhanced Raman scattering as a transduction method. The microfluidic paper-based analytical device (μPAD) is made with a lithography-free process by a simple cut and drop method. Complex samples are separated by a surface chemical gradient created by differential polyelectrolyte coating of the paper. The μPAD with a starlike shape is designed to enable liquid handling by lateral flow without microchannel patterning. This design generates a rapid capillary-driven flow capable of dragging liquid samples as well as gold nanorods into a single cellulose microfiber, thereby providing an extremely preconcentrated and optically active detection spot.

Paper-based chemical and biological sensors: Engineering aspects.

Remarkable efforts have been dedicated to paper-based chemosensors and biosensors over the last few years, mainly driven by the promise of reaching the best trade-off between performance, affordability and simplicity. Because of the low-cost and rapid prototyping of these sensors, recent research has been focused on providing affordable diagnostic devices to the developing world. The recent progress in sensitivity, multi-functionality and integration of microfluidic paper-based analytical devices (µPADs), increasingly suggests that this technology is not only attractive in resource-limited environments but it also represents a serious challenger to silicon, glass and polymer-based biosensors. This review discusses the design, chemistry and engineering aspects of these developments, with a focus on the past few years.

Gold Nanorods as Plasmonic Nanotransducers: Distance-Dependent Refractive Index Sensitivity

Owing to the facile tunability of the localized surface plasmon resonance wavelength (LSPR) and large refractive index sensitivity, gold nanorods (AuNR) are of high interest as plasmonic nanotransducers for label-free biological sensing. We investigate the influence of gold nanorod dimensions on distance-dependent LSPR sensitivity and electromagnetic (EM) decay length using electrostatic layer-by-layer (LbL) assembly of polyelectrolytes. The electromagnetic decay length was found to increase linearly with both nanorod length and diameter, although to variable degrees. The rate of EM decay length increase with nanorod diameter is significantly higher compared to that of the length, indicating that diameter is a convenient handle to tune the EM decay length of gold nanorods. The ability to precisely measure the EM decay length of nanostructures enables the rational selection of plasmonic nanotransducer dimensions for the particular biosensing application.

Single-Digit Pathogen and Attomolar Detection with the Naked Eye Using Liposome-Amplified Plasmonic Immunoassay.

We introduce an enzyme-free plasmonic immunoassay with a binary (all-or-none) response. The presence of a single pathogen in the sample results in a chemical cascade reaction leading to a large red to dark-blue colorimetric shift visible to the naked eye. The immediate and amplified response is initiated by a triggered breakdown of cysteine-loaded nanoliposomes and subsequent aggregation of plasmonic gold nanoparticles. Our approach enabled visual detection of a single-digit live pathogen of Salmonella, Listeria, and E. coli O157 in water and food samples. Furthermore, the assay allowed a naked-eye detection of target antibody concentrations as low as 6.7 attomolar (600 molecules in 150 μL); six orders of magnitude lower than conventional enzyme-linked immunosorbent assay (ELISA).

Interface design and multiplexed analysis with surface plasmon resonance (SPR) spectroscopy and SPR imaging

Ever since the advent of surface plasmon resonance (SPR) and SPR imaging (SPRi) in the early 1990s, their use in biomolecular interaction analysis (BIA) has expanded phenomenally. An important research area in SPR sensor development is the design of novel and effective interfaces that allow for the probing of a variety of chemical and biological interactions in a highly selective and sensitive manner. A well-designed and robust interface is a necessity to obtain both accurate and pertinent biological information. This review covers the recent research efforts in this area with a specific focus towards biointerfaces, new materials for SPR biosensing, and novel array designs for SPR imaging. Perspectives on the challenges ahead and next steps for SPR technology are discussed.

Directed assembly of gold nanorods using aligned electrospun polymer nanofibers for highly efficient SERS substrates

Nonspherical metal nanoparticles are very attractive plasmonic nanostructures owing to the facile tunability of the plasmonic properties and the presence of sharp corners and edges, which act as electromagnetic hot spots for surface enhanced Raman scattering (SERS). However, such anisotropic nanostructures exhibit strong polarization dependence in their plasmonic properties, exhibiting significantly higher SERS intensity in certain orientations. In this paper, we demonstrate a facile strategy to achieve directed assembly of aligned gold nanorods using highly aligned electrospun nanofibers. We believe that the interstices between the nanofibers act as micro-and nanochannels, resulting in hydrodynamic drag forces on the gold nanorods, thus inducing massive alignment of the same on the nanofibers. Apart from exhibiting nearly 50 times higher SERS intensity compared to a planar SERS substrate with randomly oriented nanorods, our results highlight the importance of the orientation of anisotropic nanostructures. Finite difference time domain (FDTD) simulations employed to understand the electromagnetic field distribution around an aligned nanorod array showed excellent agreement with the experimental observations.

Covalent attachment of trypsin on plasma polymerized allylamine

This paper focuses on the immobilization of a proteolytic enzyme, trypsin, on plasma polymerized allylamine (ppAA) films. The later have been deposited onto silicon substrate by means of radiofrequency glow discharge. The covalent attachment of the enzyme was achieved in three steps: (i) activation of the polymer surface with glutaraldehyde (GA) as a linker, (ii) immobilization of trypsin and (iii) imino groups reduction treatment. The effects and efficiency of each step were investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Fluorescent spectroscopy was used to evaluate the change of the biological activity following the immobilization steps. The results showed that enzyme immobilization on GA-modified substrate increases the enzyme activity by 50% comparing to adsorbed enzymes, while the imino reduction treatment improves the enzyme retention by about 30% comparing to untreated samples. In agreement with XPS and AFM data, UV-vis absorption spectroscopy, used to quantify the amount of immobilized enzyme, showed that allylamine plasma polymer presents a high adsorption yield of trypsin. Although the adsorbed enzymes exhibit a lower activity than that measured for enzymes grafted through GA linkers, the highest catalytic activity obtained was for the enzymes that underwent the three steps of the immobilization process.

Dual detection of nitrate and mercury in water using disposable electrochemical sensors.

Here we report a disposable, cost effective electrochemical paper-based sensor for the detection of both nitrate and mercury ions in lake water and contaminated agricultural runoff. Disposable carbon paper electrodes were functionalized with selenium particles (SePs) and gold nanoparticles (AuNPs). The AuNPs served as a catalyst for the reduction of nitrate ions using differential pulse voltammetry techniques. The AuNPs also served as a nucleation sites for mercury ions. The SePs further reinforced this mercury ion nucleation due to their high binding affinity to mercury. Differential pulse stripping voltammetry techniques were used to further enhance mercury ion accumulation on the modified electrode. The fabricated electrode was characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electrochemistry techniques. The obtained results show that the PEG-SH/SePs/AuNPs modified carbon paper electrode has a dual functionality in that it can detect both nitrate and mercury ions without any interference. The modified carbon paper electrode has improved the analytical sensitivity of nitrate and mercury ions with limits of detection of 8.6µM and 1.0ppb, respectively. Finally, the modified electrode was used to measure nitrate and mercury in lake water samples.