2-1-1 Introduction
Emerging contaminants have existed in the environment for decades, but their implications to the ecosystem and human beings are not attracted much attention until now.[30]
Recently, scientists found that some chemicals in the environment have high potential to interfere the endocrine system, which called “endocrine disrupters” or “endocrine disrupting chemicals” (EDCs). EDCs can be defined as an exogenous agent that interferes the synthesis, secretion, transport, bind, action, or elimination of natural hormones in the body that are responsible for the maintenance of homeostasis, reproduction, development, and behavior.[31] Several environmental pollutants are referred to as EDCs by the Environmental Agency of Japan. All people are exposed to chemicals with estrogenic effects in their everyday life, because endocrine disrupting chemicals are found in low doses in literally thousands of products. Chemicals commonly detected in people include alkylphenols, bisphenol A (BPA), dichlorodiphenyltrichloroethane (DDT), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and a variety of Phthalates. Table 2-1 shows the list of common types of EDCs.
EDCs are highly toxic and carcinogenic. They remain in the environment for a long time due to their stability and bioaccumulation. The effects associated with the presence of EDCs in the environment are: (1) increases in the breakage of eggs of birds, fishes and turtles, (2) feminization of male fishes, (3) problems in the reproductive systems in fishes, reptiles, birds and mammals, and (4) changes in the immunologic system of marine mammals.[19]
Moreover, EDCs affect the function of the endocrine system by binding to nuclear receptors.[30] The effects of EDCs in human beings reported so far have been: (1) reduction
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of the amount of sperm, (2) increases in the incidences of breast cancer, (3) testicle and prostate cancers, and (4) the endometriosis.[19] Therefore, EDCs are of great concern because of their potential in altering the normal endocrine function and physiological status of organism.
Table 2-1 List of some chemical compounds assorted as EDCs.[32]
Type Use/Origin
Dichlorodiphenyltrichloroethane (DDT)
Insecticide
Polychlorinated biphenyls (PCBs)
• A class of chlorinated compounds
• Heat medium, non-carbon paper, electric product, industrial coolants and lubricants
Bisphenol A (BPA)
• Raw material for resins
• Found in some plastic water and baby bottles, plastic food containers, dental materials, and the linings of metal food and infant formula cans
Polybrominated diphenyl ethers (PBDEs)
• A class of compounds found in flame retardants
• Plastic cases of televisions and computers, electronics, carpets, lighting, bedding, clothing, car components, foam cushions and other textiles
Phthalates
• Found in some soft toys, flooring, medical equipment, cosmetics and air fresheners
Alkylphenols Degradation products from nonionic detergents
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2-1-2 Environmental monitoring for Bisphenol A
BPA is a commonly used name for 2,2-(4,4-dihydroxydiphenyl) propane. Among EDCs, BPA is the most frequently detected compounds.[33] BPA is found to be bound to estrogen receptors and causes a weak estrogenic activity in animal experiments. It is a synthesis monomer used in the production of the production of epoxy resins, polysulfones and polycarbonates plastic such as baby bottles; and is one of the highest production synthetic compounds worldwide.[34] At manufacturing and processing facilities, low levels of BPA are directly released to surface waters and the atmosphere via permitted discharges. Therefore, BPA is often contained in environmental water and now is attracting attention. The European Union has assessed a tolerance daily intake (TDI) of BPA at 0.01 mg/g/day, and Japan assessed its TDI of BPA at 0.05 mg/kg/day.[35]
A number of physical and chemical methods for treating wastewater containing BPA have been reported. BPA was efficiently removed and only simple short-chain aliphatic acids were left. Additionally, various techniques appear to effectively aid the monitoring of BPA from wastewater. The useful technique for analyzing BPA in environmental samples has been gas chromatography-mass spectrometry (GC-MS).[11-13] High performance liquid chromatography (HPLC) has also been used, especially with toxicity test samples and atmospheric samples.[1-10] Table 2-2 shows the reported analytical method have been employed for the determination of BPA. These traditional techniques are expensive, time-consuming, sample pre-treatment and skilled technicians needed for operation, thus limiting their practical application in-situ detection.[14] Hence, it is highly desirable to develop a novel approach which is easy and rapid for in-situ and on-site environmental monitoring of BPA without costly instruments.
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Table 2-2 Reported analytical methods have been employed for the determination of BPA.
Method Detector Detection limit Reference
LC
Electrochemical Detector 2-3.6 ng/L [1,2]
UV spectrophotometer 6-1000 ng/L [3-5]
Fluorescence 3.3-25 ng/L [6-8]
Mass Spectrometry 5-10 ng/L [9,10]
GC Mass Spectrometry 1-6 ng/L [11-13]
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2-2 Sensor devices
Sensor is an analytical device that utilizes chemically or biologically responsive sensing layers to recognize a change in chemical or biological parameters of measured environment and to convert this information into an analytically useful signal. Development of sensors with new capabilities is driven by the ever-expanding monitoring needs of a wide variety of species in gases and liquids.[36] An ideal sensor should have high sensitivity, high specificity, quick response, in-situ analysis and portable capability. Figure 2-1 illustrates the concept of the representation of a sensor. In general, a sensor consists of two main components:
sensing materials and the transducer. In a sensing system, analytes can be detected by sensing materials, and then converted to the readable singles with transducer.
Figure 2-1 Concept of the representation of a sensor.
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2-2-1 Sensing materials
Sensing materials contain an appropriate indicator which changes its properties in dependence on the analyte. A sensing material in a sensor device is applied onto a suitable physical transducer to convert a change in a property of a sensing material into a suitable physical signal.[36] The key component of most sensors is the recognition element. It is also commonly referred as the “selective layer”. This layer interacts with the analyte to be detected, thereby encountering a characteristic change in one of its physical properties, such as mass, refractive index, light absorbance, reduction potential, etc.[37] A selective chemical signal which resulted from the binding process of the analyte to the recognition element, is subsequently converted into an electrical signal, amplified, and then transformed into a measurable format. The individual sensing elements only need to possess sufficient differential selectivity to yield a unique response pattern for each analyte.[38]
2-2-2 Transducers
After specific recognition elements receive reactive message, there would a segment that convert the molecular recognition event into a quantifiable signal. The signal obtained from a single transducer or an array of transducers is further processed to provide useful information about the identity and concentration of species in the sample.[36] In other words, transducer was the signal transistor, which transforms the physical property into the final readout.[37] Signal transduction has been accomplished with electrochemical, field-effect transistor, optical absorption, fluorescence, surface plasmon resonance and other devices.
The individual properties of different detection list in Table 2-3.
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Table 2-3 Comparison of different detection methods.
Detected method Properties
Electrochemical
• Detect the molecular changes by bonding target
• Need electro-activity
• Add electron mediator necessary
Weight
• Signals are detected from the target bond and then caused the weight change of the wafer surface
• Highly sensitive
Optical
• Identify molecular by optical detection (fluorescence)
• Impacted easily by external interference fluoresces
• Fluorescent materials has photo-bleaching
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2-3 Molecularly imprinted polymer (MIP)
Molecular imprinting is a kind of nano-manipulating methods which patterns polymer matrixes with the size, shape and chemical functionality similar to those of target templates.
It is a useful method for synthesizing tailor-made artificial receptors, while has a relationship between a lock and the key. In recent years, molecularly imprinted polymer (MIP) has been realized a potential method for the design and development for artificial materials with improved molecular recognition capacities.[39] In 1894, Fisher firstly developed the
“lock-and-key” notion, which has become one of the most frequently mentioned concepts.[40]
Figure 2-2 is the principle of molecular imprinting by Fischer’s “lock-and-key” concept.
Figure 2-2 Molecular imprinting by Fischer’s lock-and-key concept.
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2-3-1 Strategy of molecularly imprinted polymer
MIP is a new kind of materials with the prearrangement of structure for specific molecular recognition ability. The main compositions of MIP are template, cross-linker and functional monomer. Commonly, MIP is prepared by allowing a network polymer to form in presence of a template. The fabrication of MIP consists of three main steps: (1) pre-arrangement of the monomers around the target molecule, (2) polymerization in the presence of cross-linker, and (3) removal of the target molecule by extraction process.[41] A template molecule interacts with an appropriate functional monomer to establish specific interactions. The subsequent removal of template in the material remains many cavities which is complementary to the template in terms of sizes and shapes. The imprinted cavities have suitable size, shape, and chemical environment to selectively bind the target molecules.[42] In essence, a molecular “memory” is imprinted on the polymer, which is now capable of selectively rebinding the template.[43] The synthesis of MIP and the imprinting process are represented in Figure 2-3.
Figure 2-3 Synthesize process of MIP.[43]
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MIP can be stable in various critical chemical and physical conditions such as good thermal and mechanical for a long time and reused without any alternation to the memory of the template.[41] MIP is receiving increasing attention owing to their high selectivity and affinity for the target molecules. In the past few years, the applications of MIP were expanded to the environmental field for enrichment and separation of target compounds in the sensing and catalytic systems. There are great hopes for the development of a new generation of chemical sensors using these novel synthetic materials as recognition elements.[44] The bio-mimic MIP has a number of potential advantages including high molecular selectivity, molecular memory, the simplicity of their preparation and stability under harsh environment such as pH, humidity and temperature.
2-3-2 Imprinted methods
Essentially, there are two imprinting methods to synthesize MIP. The methods of imprinting are according to interactions between functional monomers and template compounds. In general, the association between the imprinted molecules and monomers is generally based on non-covalent and covalent interactions. Figure 2-4 shows the ordinary preparation of the covalent and non-covalent imprinting processes.[45]
Covalent imprinting refers to molecular imprinting strategies whereby the template and one or more polymerization units are attached by covalent bonds to form a
“template-monomer”. The classical methods of covalent imprinting involve boronate esters, metal coordination, acetal/ketal, Schiff base formation and imines to prepare template-monomers.[46] Covalent approach utilizes reversible covalent bonding between a polymerizable monomer and a template molecule. After polymerization, these bonds are cleaved to liberate the template, but they are subsequently reformed in order to selectively
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bond the target.[41] Although the stronger covalent binding is facile to generate the rigidity and homogeneity of binding site matrix, the subsequent extraction of template is more difficult to reach. Because the covalent imprinting technique leads to strong interactions between the matrix and the target, it is limited by the relatively small number of useful reversible covalent bonding reactions that can be utilized.
Figure 2-4 Representation of the covalent and non-covalent imprinting processes.[45]
The non-covalent interactions include hydrogen bonds, ionic bonds, hydrophobic interactions, electrostatic attraction and Van der Waals’ forces, etc. After the polymerization and removal of the template, the functional groups of the polymeric matrix can then rebind the target via the same non-covalent interactions. Unlike those used in covalent imprinting, the fragile interaction is facilitated extraction or elution in the following procedure and easily obtained. Since non-covalent method is facile to adapt, it is popularly employed in most molecularly imprinting processes.[41] However, the weak interactions limit that the template and target must form a sufficient number of non-covalent intermolecular interactions to generate binding pockets during polymerization. The different effects of covalent and
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non-covalent interactions on each imprinting steps are listed in Table 2-4.
Table 2-4 Comparison of covalent and non-covalent imprinting.[41]
Terms Covalent Non-covalent
Pre-polymerization Need some synthetic chemistry No need Extraction of
template
Difficult Easy
Rebinding rate Slow Rapid
Binding sites Homogeneous of receptor sites Heterogeneous of receptor sites
Advantages
• Stoichiometriy help lower non-specific interaction
• High affinity receptor sites are easy produced
• Functional groups can be targeted widely
• The preparation process is simply
Disadvantages
• More complex process
• Less functional groups can be used
• Lower template recovery
• Apparent non-specific binding
• Low yield of functional high affinity receptor sites
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2-4 Photonic crystal
Photonic crystal is a highly ordered material that possesses periodically modulated dielectric constants. Periodicity affects the propagation of electromagnetic waves in the material due to Bragg reflections on lattice planes and results a photonic band gap (PBG) in which light propagation in the photonic crystal is forbidden.[47]
2-4-1 Principle of photonic crystal
The concept of photonic crystal is a relatively new class of materials which was firstly proposed by Yablonovich and John in 1987.[48] Many researches on photonic crystal have been extended to cover all three dimensions, and the spectral range has been extended from ultraviolet to radio frequencies. Photonic crystal is in one-dimensional, two-dimensional or three-dimensional highly order structures with periodical dielectric constants. Figure 2-5 exhibits different dimensional periodicities in a medium. The one-dimensional version of a photonic crystal has long been known as a multilayer reflector, which has been widely used in optical lenses. And two-dimensional or three-dimensional structure of photonic crystal are the most currently important aspect in the field of photonic crystal.[49]
Figure 2-5 Models for one-dimensional, two-dimensional and three-dimensional periodicities in a medium.[50]
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The major feature of photonic crystal is the existence of a band gap in its photonic structure that is able to influence the propagation of electromagnetic waves in a similar way as the electronic band gap of a semiconductor does for electrons. Light with frequencies within the band gap cannot propagate within the photonic crystal materials. The photonic crystal will completely diffract incident light in accordance with Bragg’s law (Eq. 2-1).[51]
m 2 d
111( n
2ef sin
2)
1/2 Eq. 2-1m: the order of diffraction light λ: Bragg diffraction wavelength d111: the spacing between (111) planes
nef: the mean refractive index of the crystalline lattice
θ: the angle between the incident light and the normal to the diffraction planes
Figure 2-6 indicates the refraction of an incident light in photonic crystal. The reflected light in the visible spectrum is called the structural color. Additionally, the optical characteristics of photonic crystal are related to their lattice constants. The other important properties of photonic crystal are ordered structure and anisotropy. Photonic crystal materials provide the opportunity to control the flow of light. The optical properties of these periodic media are determined by the interference of multiply diffracted waves, and therefore are very sensitive to the material parameters such as the refractive index and lattice spacing.
This sensitivity can be exploited for the purposes of optical sensing, in a number of different ways.
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Figure 2-6 Scheme of the optical path of incident light in photonic crystal.[50]
2-4-2 Preparation of photonic crystal
Recently, scientists have discovered that order porous materials with the aperture close to an optical wavelength exhibited special optical properties and can be applied in photocatalysis, adsorption, filtration, and sensors. Top-down lithography and etching processes have been utilized to prepare three-dimensional photonic crystal. However, complex procedures and expensive facilities are required to achieve desired structures.
Organisms have an ability of self-assembling that can self-generate hair, teeth and bones. If the preparation simulates such molecular pattern to fabricate structure hierarchically, it would be easier relatively to obtain regularity layer-by-layer. The method also commonly called bottom-up approach. The self-assembly of colloidal crystals is the more preferred route as it is simpler, inexpensive and can yield crystalline samples of a few to several hundred structural layers thinness. In order to manufacture three-dimensional photonic crystal, more research had focused on the self-assembling recently.
Polystyrene (PS), a polymethyl methacrylate and silica microspheres, is the major particle that usually used for the colloidal crystal assembly, as they can be obtained both highly monodisperse and relatively cheap.[52] Based on the difference in structure, photonic
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crystal can be divided into two categories: opal structure and inverse opal structure. Natural opal is an example of such a periodic colloidal crystal, and is composed of face-centered cubic (fcc) arrays of monodisperse amorphous silica spheres with average diameter in the range 15-900 nm.[47] Opals are formed when the voids between ordered sediments of the colloidal crystal are infiltrated by media having a high refractive index and then solidified.
The colloidal crystals are subsequently removed by calcination or solvent extraction, leaving behind a new material with pores that referred as inverse opal structure. The long-range ordering of particles in the structure of photonic crystal results in a number of unique potentially useful properties.
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2-5 Sensing applications
2-5-1 Photonic crystal sensor
Photonic crystal sensors utilize different fundamental properties of the optical signal including amplitude, frequency, polarization and phase. It results in a vast number of sensor designs such as intensity-based, spectroscopic, polarimetric and interferometric.[53]
Numerous examples of photonic crystal sensors are documented. The most common sensors are based on changes in intensity due to absorption, reflection, emission, scattering, fluorescence and surface plasmon resonance.[54] Three-dimensional opal and inverse opal structures have been demonstrated to optically determine analyte molecules by the shift in the Bragg’s diffraction wavelengths.[23-26]
In 1994, Asher et al.[26] developed a PS colloidal crystalline array which was filled with a hydrogel within its interstitial space. The composite swelled and shrank reversibly in the presence of metal ions and the quantities of the analyses were determined by the changes in the lattice spacing of the colloidal crystals. Qian et al.[25] designed a biosensor based on three-dimensional PS substrate and measured the changes in the refractive index during analyte binding. Endo et al.[55] constructed a colloidal crystal-based chemical sensor with a reversibly tunable structural color for volatile organic compounds (VOCs) detection. The device consisted of a glass substrate with three-dimensional colloidal crystal and poly-dimethylsiloxane (PDMS) elastomer that was capable of swelling after rebinding pollutants. Reese et al.[56] built a colorimetric reagent that particles composed of an intelligent polymerized crystalline colloidal array for the determination of Pb2+, pH and temperature. If the pH, ionic strength or temperatures change, the diffracted wavelength of the polymerized crystalline colloidal array will shift due to changes in the volumes. Those sensors and biosensors can be direct, measuring the intrinsic properties of the analyte.
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2-5-2 Molecular imprinted polymer
The obvious advantages of MIP are exhibited a highly specific recognition ability and a selective adsorption toward analyte. MIP have been designed not only as molecular recognition materials but also as sensing materials that couple a readable signal to a binding event, which can then be used to directly detect and quantify the target analyte. When the analytes have a chromophore or a fluorophore, the binding events can be read out if these compounds show any spectral changes due to the binding. An aspect in design of MIP based sensor is important due to the low price, chemical stabile and high selectivity.[57]
Marx et al.[58] designed a thin film that was a molecularly imprinted sol-gel polymer with specific binding sites for parathion. Vandevelde et al.[59] used fountain pen microlithography to deposit arrays of MIP microdots on flat substrates. It was able to show analyte binding to the dots by fluorescence microscopy with the aid of a fluorescent model analyte. Murray et al.[60] developed a sensor that was able to selectively measure the hydrolysis product of the
Marx et al.[58] designed a thin film that was a molecularly imprinted sol-gel polymer with specific binding sites for parathion. Vandevelde et al.[59] used fountain pen microlithography to deposit arrays of MIP microdots on flat substrates. It was able to show analyte binding to the dots by fluorescence microscopy with the aid of a fluorescent model analyte. Murray et al.[60] developed a sensor that was able to selectively measure the hydrolysis product of the