Surface plasmon resonance (SPR) has found extensive applications in chemi-sensors and biosensors. Plasmons play different roles in different types of optical sensors. SPR transduces a signal in a colorimetric sensor through shifts in the spectral position and intensity in response to external stimuli. SPR can also concentrate the incident electromagnetic field in a nanostructure, modulating fluorescence emission and enabling plasmon-enhanced fluorescence to be used for ultrasensitive detection. Furthermore, plasmons have been extensively used for amplifying a Raman signal in a surface-enhanced Raman scattering sensor. This paper presents a review of recent research progress in plasmon-enhanced optical sensing, giving emphasis on the physical basis of plasmon-enhanced sensors and how these principles guide the design of sensors. In particular, this paper discusses the design strategies for nanomaterials and nanostructures to plasmonically enhance optical sensing signals, also highlighting the applications of plasmon-enhanced optical sensors in healthcare, homeland security, food safety and environmental monitoring.

Plasmon-enhanced optical sensors: a review

Prostanoids are cyclic, oxygenated products of ω3 and ω6 20- and 22-carbon essential fatty acids (FAs) that are formed enzymatically through “cyclooxygenases”. Prostaglandin endoperoxide synthases -1 and -2 (PGHS-1 and -2)a, which are also known as cyclooxygenases -1 and -2 (COX-1 and -2), catalyze the committed step in the biosynthesis of prostanoids (Figure 1). These compounds include what are sometimes referred to as the “classical” prostaglandins (PGs) PGD, PGE, and PGF as well as prostacyclins denoted as PGI's and the thromboxanes abbreviated Tx's; monohydroxy acids can also be formed via PGHSs, but information on the possible physiologic importance of such compounds is incomplete.1-3 The most abundant prostanoids are the “2-series” compounds (e.g. PGE2) that are formed from arachidonic acid (AA; 5Z, 8Z, 11Z, 14Z- eicosatetraenoic acid; 20:4 ω6; Figure 1). The “2” denotes the number of carbon-carbon double bonds in the product.



Figure 1

Biosynthetic pathway for the formation of prostanoids



PGHSs catalyze two distinct reactions that occur at physically distinct but functionally interacting sites. The cyclooxygenase (COX) reaction is a bis-oxygenation in which two O2 molecules are inserted into the carbon backbone of AA to yield PGG2 (Figure 1). The peroxidase (POX) reaction is a transformation in which the 15-hydroperoxyl group of PGG2 undergoes a net two electron reduction to PGH2 plus water. The POX reaction is important in the enzyme mechanism, but other peroxidases such as glutathione peroxidase may contribute importantly to the reduction of PGG2 to PGH2 in vivo.

PGH2 is thought not to accumulate in cells but rather to be converted quickly to what are considered the biologically relevant, downstream products. There are specific synthases involved in forming PGD2, PGE2, PGF2α, PGI2 and TxA2 from PGH2. Except for the case of PGF2α, which is formed by a two electron reduction of PGH2, these enzymes catalyze non-oxidative rearrangements. Finally, there is a PGH 19-hydoxylase that converts PGHs to their corresponding 19-hydroxy derivatives which themselves are substrates for PGE synthase(s). There are one or more specific G protein-linked receptors for each prostanoid, and in some cases prostanoids may also act through nuclear peroxisome proliferator activated receptors.

AA and other 20- and 22-carbon, highly unsaturated FAs are found esterified at the sn2 position of glycerophospholipids. Basal prostanoid formation generally occurs at a low rate relative to stimulated formation. A major factor limiting prostanoid formation is AA availability, which is controlled through the net rates of deacylation and reacylation of glycerophospholipids. Prostanoid formation is enhanced when phospholipase A2 (PLA2) activity is increased, and thus PLA2s play a substrate-limiting role in regulating prostanoid biosynthesis. Although reacylation may also be important, its possible role in regulating prostanoid biosynthesis is largely unexplored.

In this chapter we review the biochemistry and the biochemical pharmacology of the enzymes involved in converting AA to various prostanoid products. These enzymes include PGHS-1, PGHS-2, hematopoietic PGD synthase (H-PGDS), lipocalin-type PGD synthase (L-PGDS), microsomal PGE synthase-1 (mPGES-1), microsomal PGE synthase-2 (mPGES-2), cytosolic PGE synthase (cPGES), PGF synthase (PGFS), PGI synthase (PGIS) and TXA synthase (TXAS). The PLA2s involved in mobilizing AA and the receptors through which prostanoids function are surveyed in other chapters of this volume.

Enzymes of the Cyclooxygenase Pathways of Prostanoid Biosynthesis

Optical biosensors have exhibited worthwhile performance in detecting biological systems and promoting significant advances in clinical diagnostics, drug discovery, food process control, and environmental monitoring. Without complexity in their pretreatment and probable influence on the nature of target molecules, these biosensors have additional advantages such as high sensitivity, robustness, reliability, and potential to be integrated on a single chip. In this review, the state of the art optical biosensor technologies, including those based on surface plasmon resonance (SPR), optical waveguides, optical resonators, photonic crystals, and optical fibers, are presented. The principles for each type of biosensor are concisely introduced and particular emphasis has been placed on recent achievements. The strengths and weaknesses of each type of biosensor have been outlined as well. Concluding remarks regarding the perspectives of future developments are discussed.

Optical biosensors: an exhaustive and comprehensive review.

Plasmonic nanoparticles can strongly interact with adjacent fluorophores, resulting in plasmon-enhanced fluorescence or fluorescence quenching. This dipolar coupling is dependent upon nanoparticle composition, distance between the fluorophore and the plasmonic surface, the transition dipole orientation, and the degree of spectral overlap between the fluorophore’s absorbance/emission and the surface plasmon band of the nanoparticles. In this work, we examine the distance and plasmon wavelength dependent fluorescence of an infrared dye (“IRDye”) bound to silica-coated gold nanorods. Nanorods with plasmon band maxima ranging from 530 to 850 nm are synthesized and then coated with mesoporous silica shells 11–26 nm thick. IRDye is covalently attached to the nanoparticle surface via a click reaction. Steady-state fluorescence measurements demonstrate plasmon wavelength and silica shell thickness dependent fluorescence emission. Maximum fluorescence intensity, with approximately 10-fold enhancement, is observed ...

Distance and Plasmon Wavelength Dependent Fluorescence of Molecules Bound to Silica-Coated Gold Nanorods

Metal enhanced fluorescence (MEF) for biosensors: General approaches and a review of recent developments

Enhancing the fluorescence of a weak emitter is important to further extend the reach of single-molecule fluorescence imaging to many unexplored systems. Here we study fluorescence enhancement by isolated gold nanorods and explore the role of the surface plasmon resonance (SPR) on the observed enhancements. Gold nanorods can be cheaply synthesized in large volumes, yet we find similar fluorescence enhancements as literature reports on lithographically fabricated nanoparticle assemblies. The fluorescence of a weak emitter, crystal violet, can be enhanced more than 1000-fold by a single nanorod with its SPR at 629 nm excited at 633 nm. This strong enhancement results from both an excitation rate enhancement of ∼130 and an effective emission enhancement of ∼9. The fluorescence enhancement, however, decreases sharply when the SPR wavelength moves away from the excitation laser wavelength or when the SPR has only a partial overlap with the emission spectrum of the fluorophore. The reported measurements of fluo...

https://pure.tue.nl/ws/files/3895403/691632953887049.pdf

Resonant plasmonic enhancement of single-molecule fluorescence by individual gold nanorods

The enzyme mechanism of the multicopper oxidase (MCO) SLAC from Streptomyces coelicolor was investigated by structural (XRD), spectroscopic (optical, EPR), and kinetics (stopped-flow) experiments on variants in which residue Tyr108 had been replaced by Phe or Ala through site-directed mutagenesis. Contrary to the more common three-domain MCOs, a tyrosine in the two-domain SLAC is found to participate in the enzyme mechanism by providing an electron during oxygen reduction, giving rise to the temporary appearance of a tyrosyl radical. The relatively low k(cat)/K(M) of SLAC and the involvement of Y108 in the enzyme mechanism may reflect an adaptation to a milieu in which there is an imbalance between the available reducing and oxidizing co-substrates. The purported evolutionary relationship between the two-domain MCOs and human ceruloplasmin appears to extend not only to the 3D structure and the mode of binding of the Cu's in the trinuclear center, as noted before, but also to the enzyme mechanism.

Involvement of Tyr108 in the enzyme mechanism of the small laccase from Streptomyces coelicolor

Rice α-oxygenase (RαO) catalyzes the insertion of O2 into the Cα−H bond of various fatty acids. The mechanism is thought to involve a tyrosyl radical as the oxidant on the basis of comparisons to the structurally homologous cyclooxygenase enzymes. Kinetic and spectroscopic results presented here for the wild-type RαO and the Tyr379Phe mutant indicate an irreversible H• abstraction mechanism and support the involvement of the proposed catalytic Tyr•. In addition, very large, weakly temperature dependent deuterium kinetic isotope effects (∼50) are observed, consistent with extensive nuclear tunneling. RαO, thus, presents a novel example where such quantum effects are associated with an amino acid radical-utilizing enzyme.

Evidence for Protein Radical-Mediated Nuclear Tunneling in Fatty Acid α-Oxygenase

Plasmonic fluorescence enhancement is used to study fluorescence correlation spectroscopy (FCS) at higher concentrations than in regular diffraction-limited FCS experiments. Previous studies suffered from sticking to the substrate and were performed mainly with poorly emitting dyes. A lipid bilayer forms a passivating surface, preventing sticking of the dye or the protein, and allows for specific anchoring of probe molecules. For dyes with high quantum yields, the fluorescence background of unenhanced molecules is high, and the fluorescence enhancement is weak, less than about 10. Nonetheless, we show that FCS is possible at micromolar concentrations of the probe molecule. Enhanced FCS is recorded by selecting signals on the basis of their shortened lifetime. This selection enhances the contrast of the correlation by more than an order of magnitude. The lipid bilayer can be used to anchor biomolecules and perform enhanced FCS, as we show for a dye-labeled protein.

Gold-Nanorod-Enhanced Fluorescence Correlation Spectroscopy of Fluorophores with High Quantum Yield in Lipid Bilayers

Single-molecule enzymology provides an unprecedented level of detail about aspects of enzyme mechanisms which have been very difficult to probe in bulk. One such aspect is intramolecular electron transfer (ET), which is a recurring theme in the research on oxidoreductases containing multiple redox-active sites. We measure the intramolecular ET rates between the copper centers of the small laccase from Streptomyces coelicolor at room temperature and pH 7.4, one molecule at a time, during turnover. The forward and backward rates across many molecules follow a log-normal distribution with means of 460 and 85 s–1, respectively, corresponding to activation energies of 347 and 390 meV for the forward and backward rates. The driving force and the reorganization energy amount to 0.043 and 1.5 eV, respectively. The spread in rates corresponds to a spread of ∼30 meV in the activation energy. The second-order rate constant for reduction of the T1 site amounts to 2.9 × 104 M–1 s–1. The mean of the distribution of for...

http://laysanbio.com/clientuploads/JACS%20paper%20136%202707.pdf

Ankur Gupta

Papers

Resonant plasmonic enhancement of single-molecule fluorescence by individual gold nanorods

Involvement of Tyr108 in the enzyme mechanism of the small laccase from Streptomyces coelicolor

Evidence for Protein Radical-Mediated Nuclear Tunneling in Fatty Acid α-Oxygenase

Gold-Nanorod-Enhanced Fluorescence Correlation Spectroscopy of Fluorophores with High Quantum Yield in Lipid Bilayers

One at a time: intramolecular electron-transfer kinetics in small laccase observed during turnover.