Surface plasmon resonance for water pollutant detection and water process analysis
Introduction
Hazardous materials, such as heavy metals, pharmaceutical compounds, and pathogens, are excessively discharged into water environment, leading to water pollution and serious threats to public health. Many analytical approaches, including UV–visible spectrometry, fluorescence, chromatography, mass spectrometry (MS), and atomic absorption spectrometry, have been applied for the detection of environmental pollutants.
On the one hand, appearance of various trace and/or emerging pollutants has challenged the existing analytic methods, which typically can determine only specific types of analytes. On the other hand, the efficiency of water treatment should be improved to reduce energy consumption or even produce energy. For example, antifouling materials can be employed on the membrane surface to decrease fouling and to keep flux in the membrane separation process. Additional, revealing the metabolic pathway for pollutant degradation is important to enhance the removal via biological wastewater treatment. These requirements are excellently met by a valuable and powerful tool, surface plasmon resonance (SPR), which has been widely used not only for studying environmental pollutants with a broad range of object dimensions from ions to cells but also for characterizing the performance of antifouling agents and probing microbial adhesion and growth events.
The Kretchmann configuration is most commonly adopted to describe the principle of SPR sensing (Fig. 1a). The major components of this optical system consist of an excitation light source, a prism and a metallic film (~50 nm) coated on the prism or glass. When an incident light of p-polarization is shined from an optically denser medium (glass) into optically thinner medium (gas or water), total reflection will occur with the incident angle exceeding a critical value. The incident light produces an evanescent wave that penetrates into the thinner medium with a depth of approximately half of the wavelength, and then returns to the denser medium.
When a thin metallic film exists between the glass layer and the water layer, the free electrons in the metal surface can be excited by the incident light to form surface plasmon wave that propagates parallel to the metal surface. SPR is generated when the evanescent wave couples with the surface plasmon wave, which changes the total reflection condition and an attenuated total reflection emerges. As a result, the reflected light energy dramatically decreases due to the transfer of the incident energy to the surface plasmon. A minimum in the reflected light intensity occurs at an angle that corresponds with the incident angle, and this angle is defined as the SPR or resonance angle (θSPR). The SPR angle shifts when a molecular monolayer is formed on the metal surface in the evanescent field at a typical depth of ~200–300 nm. Importantly, this SPR angle shift is ultrasensitive to the refraction index or the mass density of the monolayer close to the metal surface. Therefore, a reaction on the chip surface can be monitored in real time. The SPR angle shift is described in terms of resonance units (RU), where an angle shift of 0.1° corresponds to 1000 RU [1].
SPR spectrometry provides an analysis method that is label-free, real-time, rapid and sensitive, and it consumes minimal sample. It has been widely used to acquire the binding specificity between two molecules, a target molecule's concentration, the kinetic parameters of association and dissociation processes, the binding affinity, cell adhesion and migration, and so on. Fig. 1c illustrates typical SPR data monitoring the dynamic adsorption of macromolecules onto the SPR chip surface in real time [2]. Buffer is flowed over the chip surface using an appropriate flow rate until the baseline stabilizes (Fig. 1c, I). A macromolecule sample is then injected in the flow cell, which then adsorbs onto the chip surface (Fig. 1c, II). The adsorption and desorption processes eventually reach an equilibrium state (Fig. 1c, III). Finally, the dissociation process is initiated by buffer injection to remove unbound or loosely bound macromolecules (Fig. 1c, IV). The SPR signal (RU) versus time is recorded during the entire process.
LSPR is created when incident wavelength matches with the collective oscillations of the electron excited by the incident light in noble metal nanostructures [3]. The resonance generates sharp peaks in the extinction spectrum that are sensitive to the dielectric of the medium on the surface of the nanostructure. Thus, LSPR can recognize and characterize macromolecules without prism configuration.
Different from the conventional SPR, LRSPR is generated when a buffer medium is introduced between the prism and the metallic layer, and in which the refractive index is similar to the analyte. Compared to conventional SPR, LRSPR sensors have significantly improved sensitivity and applicability due to their sharper angular resonance curve, deeper depth of penetration (~1000 nm), and higher resolution of refractive index [4].
Ligand immobilization on a gold substrate is normally required to recognize a specific receptor, i.e. an environmental pollutant, macromolecule or cell, when using SPR chip sensing. Fig. 2 shows the common strategies to modify the chip gold surface or directly anchoring probes on it.
Physical adsorption is a simple approach, which attaches the target substance on a chip as a result of hydrogen bonding, van der Waals forces, electrostatic forces, and hydrophobic interactions (Fig. 2a). Hydrophobic head groups or thin layer modification of the chip surface can adsorb hydrophobic ligands or its moieties. It is noted that the attachment layer may have a disordered orientation, may be weakly bound, and may have a low packing density. These factors may in turn decrease the probes activity [5].
Ligand with a thiol group (R–SH) can strongly absorb onto the gold chip surface via an Au–S bond, resulting in a homogeneous orientation (Fig. 2b). Ligands that lack the thiol group can use chemical synthesis and protein engineering to introduce thiol groups [6].
Self-assembled monolayer (SAM)-based immobilization is the most studied method to date. A SAM is an ordered molecular structure formed via the adsorption of an active surfactant onto a solid surface in dilute solution [7]. Thiolate monolayers on Au are the most extensively studied SAMs on SPR sensor. SAM with different surface properties, such as the wettability, surface charge, and morphology, can be easily prepared by thiolates with various terminal functional groups. SAMs with terminal carboxyl groups can covalently bind with the primary amine of a ligand based on an amine coupling method using activation of the carboxyl with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide and N-hydroxysuccinimide; this approach is widely used to immobilize protein. For carbohydrate attachment, SAMs with terminal carboxyl groups can be modified with hydrazine or carbohydrazide and activated by NaCNBH4; then, the aldehyde group is introduced [8]. The detailed immobilization process of each ligand can refer to the book [9]. In the covalent immobilization process, the nonspecific binding of ligand could shroud the reactive functional groups in SAMs, and decresed the efficiency of immobilization. Furthermore, after the ligand immobilization, blocking agents, e.g. ethanolamine is used to block the residual carboxylic groups on the surface to prevent them from interacting with sample.
SAM-based polymerization, such as surface-initiated atom transfer radical polymerization (SI-ATRP) and surface-initiated photoiniferter-mediated polymerization (SI-PIMP), can graft synthesized monomer of a polymer onto the SAM-modified gold surface to form a polymer film or brush. For the SI-ATRP, the monomer is polymerized onto the SAMs as a result of the contribution of CuCl2 and Tris[2-(dimethylamino)ethyl]amine [10], whereas SI-PIMP can be performed with a monomer solution and photoiniferter SAMs via a UV lamp [11].
The Capture method is an alternative when the ligand activity is damaged or is otherwise unsuitable as a result of formation of a covalent method, immobilizing the ligand with a homogeneous orientation. First, a capture molecule that has a high affinity for the ligand is covalently immobilized on the chip, and this capture molecule subsequently leads to ligand immobilization (Fig. 2c). Streptavidin-biotin [12], antibody-antigen [13], and poly(histidine) tagged-nickel-nitrilotriacetic acid [14] capture molecules are the commonly used with high affinity in previous studies. However, the captured ligand will be removed in the chip regeneration process, and a new ligand will need to be re-immobilized, which results in excess consumption of the ligand.
A highly sensitive SPR signal can be obtained by fabricating a polymer nanolayer on the surface of the sensor chip (Fig. 2d). The polymer thin film can be formed via chip immersion in a polymer solution, a spin-coating method, a Langmuir-Blodgett (LB) technique, and other approaches. The spin-coating approach can quickly obtain thin layer in several tens of minutes [15]. The LB method can better control the thickness and homogeneity of the film, which is usually applied in the preparation of a nanoscale film [16]. Compared with the stable covalent immobilization, however, polymer films are weakly bound to the surface by physisorption [17].
Thus far, the SPR principles and surface modifications were introduced in this review. SPR application in the detection of heavy metals, organic pollutants, and bacteria, and its use in monitoring microbial attachment and antifouling processes were described.
Section snippets
Pollutant determination
The majority of pollutants in water environments such as heavy metal, organic pollutants, and pathogens, can be determined by SPR using an appropriate assay format. This work reviews the use of SPR in pollutant detection over the past decade.
Monitoring microbial attachment
Bio-adhesion, including macromolecule adhesion and cell attachment, is a biochemistry phenomenon that is widely present in water environments. In fact, bio-adhesion must be enhanced in biological wastewater treatment systems to promote microbial aggregation and biofilm formation to maintain stable reactor operation. The process of biofilm formation is described by five stages: (1) planktonic, (2) microbial attachment, (3) microcolony formation, (4) macrocolony formation, and (5) biofilm or cell
Probing of antifouling
Biofilm formation in drinking water distribution system can accelerate microbial corrosion, worsen overall integrity, reduce residual disinfectant levels, release pathogens and other bacteria, and produce secondary pollutants [76]. In membrane separation processes, adsorption and blockage caused by biomacromolecules or fine particles and biofilm growth on/in membrane pores can result in membrane fouling, which lessens treatment efficiency and membrane performance [77]. Therefore, these adhesive
Future work needs
According to previous studies, SPR has been widely used in the detection of environmental pollutants and monitoring bio-adhesion. Still, exploitation of the powerful capabilities of SPR may prove worthwhile in the following fields of water pollution process in the near future.
Conclusions
SPR provides an analysis method that is label-free, real-time, rapid, and sensitive and minimal sample consumption. For nearly a decade, SPR has been largely employed to investigate water pollution and analyse water treatment process. Most environmental pollutants, including heavy metals, organic matters, and pathogens, can be determined by SPR using the appropriate surface modification. The lowest detectable concentrations of heavy metals are as low as to pM-µM. Organic pollutants, such as
Acknowledgements
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (51578527, 51508546 and 51278509), the Public Experiment Center of State Bioindustrial Base (Chongqing) and Shanghai Tongji Gao Tingyao Environmental Science and Technology Development Foundation.
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