Polymerized Crystalline Colloidal Array Research Articles

Progress toward the development of a point-of-care photonic crystal ammonia sensor

We have developed an ammonia-sensitive material by coupling the Berthelot reaction to our polymerized crystalline colloidal array (PCCA) technology. The material consists of a periodic array of highly charged colloidal particles (110 nm diameter) embedded in a poly(hydroxyethyl acrylate) hydrogel. The particles have a lattice spacing such that they Bragg-diffract visible light. In the Berthelot reaction, ammonia, hypochlorite, and phenol react to produce the dye molecule indophenol blue in an aqueous solution. We use this reaction in our sensor by covalently attaching 3-aminophenol to the hydrogel backbone, which forms cross-links through the Berthelot mechanism. Ammonia reacts with hypochlorite, forming monochloramine, which then reacts with a pendant aminophenol to form a benzoquinone chlorimine. The benzoquinone chlorimine reacts with another pendant aminophenol to form a cross-link. The creation of new cross-links causes the hydrogel to shrink, which reduces the lattice spacing of the embedded colloidal array. This volume change results in a blue-shift in the diffracted light proportional to the concentration of NH3 in the sample. We demonstrate that the NH3 photonic crystal sensing material is capable of quantitative determination of concentrations in the physiological range (50-350 micromol NH3 L(-1)) in human blood serum.

Rapid Preparation of Crystalline Colloidal Arrays Using a Strong Electric Field Dialysis

We present a simple method for rapid preparation of crystalline colloidal arrays (CCAs) by a strong electric field dialysis (SEFD). This method is based on rapid removing of ionic impurities in colloidal suspensions by applying a strong electric field. In a SEFD process, the negatively charged ions in colloidal suspensions are rapidly driven to the anode, the positively charged ones are rapidly driven to the cathode, and the colloidal particles are withheld in the dialysis tube. It was shown that the colloidal particles aggregated on the wall of the dialysis tube could block the SEFD process, which could be overcome by reversing the direction of the electric field. The purified colloidal particles can self-assemble into a crystalline colloidal array, which has an electrostatically stabilized three-dimensional periodic array of colloidal particles with a characteristic lattice spacing that can be varied by dilution. The reflection spectra show distinct peaks due to diffraction from CCAs. Atomic force microscopy (AFM) image illustrates the non-contacted ordering of the colloidal particles in the CCAs embedded in gels. This indicates the formation of high-quality single CCAs. Using a SEFD method, the preparation time of CCAs can be reduced. This new technique will greatly speed up the process of preparing polymerized crystalline colloidal arrays (PCCAs) into real-world application in the analytical field.

Simple Nanosecond to Minutes Transient Absorption Spectrophotometer

We built a transient absorption spectrophotometer that can determine transient absorption spectral changes that occur at times as fast as approximately 200 ns and as slow as a minute. The transient absorption can be induced by a temperature-jump (T-jump) or by optical pumping from the deep ultraviolet (UV) to the infrared (IR) by use of single ns Nd:YAG laser pulses. Our use of a fiber-optic spectrometer coupled to a XeF flashlamp makes the collection of transient spectra easy and convenient in the spectral range from the near IR (1700 nm) down to the deep UV (200 nm), with high signal-to-noise (S/N) ratios. The spectral resolution is determined by the specific configuration of the fiber-optic spectrometer (grating groove density, fiber diameter, slit width) and varies between 0.3 and 10 nm. The utility of this spectrometer was demonstrated by measuring the rate at which a polymerized crystalline colloidal array (PCCA) of poly(N-isopropylacrylamide) nanogel particles optically switch light due to a T-jump induced by nanosecond 1.9 microm laser pulses. In addition, we measured the rate of optical switching induced by a 3 ns 355 nm pump pulse in PCCA functionalized with azobenzene.

Design of Functional Polymer Gels and Their Application to Biomimetic Materials

Over about the last two decades, many kinds of stimuli-responsive polymer gels in response to the change in their surroundings such as solvent composition, temperature, pH, and supply of electric field, etc., have been developed. They have attracted much attention as intelligent (or smart, biomimetic) materials which have sensor, processor and actuator functions. Applications to actuator (artificial muscle), biosensor, drug delivery systems, purification or separation systems, etc. are extensively studied. In this review, design of stimuli-responsive gels and their application to bio- or biomimetic materials will be discussed as well as our recent studies on novel self-oscillating gels. Keywords: reversible adsorption, drug delivery systems (DDS), glucose-responsive gel, polymerized crystalline colloidal arrays (PCCA), nanotechnology, microfabrication

Photoswitchable Spirobenzopyran- Based Photochemically Controlled Photonic Crystals

We have developed a photochemically controlled photonic-crystal material by covalently attaching spiropyran derivatives to polymerized crystalline colloidal arrays (PCCAs). These PCCAs consist of colloidal particles that self-assemble into crystalline colloidal arrays (CCAs), which are embedded in crosslinked hydrogels. Photoresponsive PCCAs were made two ways: 1) by functionalizing the hydrogel network with spiropyran derivatives, and 2) by functionalizing the colloidal particles with spiropyran derivatives. These materials can diffract light in the UV, visible, or near-IR spectral regions. The diffraction of the PCCAs is red-shifted by exciting the spiropyran with UV light. Alternatively, the diffraction is blue-shifted by exciting the spiropyran with visible irradiation. Thus, this material acts as a memory storage material where information is recorded by illuminating the PCCA and information is read out by measuring the photonic-crystal diffraction wavelength. UV excitation forms the open spiropyran form while visible excitation forms the closed spiropyran form. The diffraction shifts result from changes in the free energy of mixing of the PCCA system as the spiropyran is photoexcited to its different stable forms.

Modeling of Stimulated Hydrogel Volume Changes in Photonic Crystal Pb2+Sensing Materials

We modeled the stimulated hydrogel volume transitions of a material which binds Pb2+ and is used as a photonic crystal chemical sensing material. This material consists of a polymerized crystalline colloidal array (PCCA) hydrogel which contains a crown ether molecular recognition group. The PCCA is a polyacrylamide hydrogel which embeds a crystalline colloidal array (CCA) of monodisperse polystyrene spheres of approximately 100 nm. The array spacing is set to diffract light in the visible spectral region. Changes in the hydrogel volume induced by Pb2+ binding alter the array spacing and shift the diffracted wavelength. This system allows us to sensitively follow the hydrogel swelling behavior which results from the immobilization of the Pb2+ by the crown ether chelating groups. Binding of the Pb2+ immobilizes its counterions. This results in a Donnan potential, which results in an osmotic pressure which swells the hydrogel. We continue here our development of a predictive model for hydrogel swelling based on Flory's theory of gel swelling. We are qualitatively able to model the PCCA swelling but cannot correctly model the large responsivity observed at the lowest Pb2+ concentrations which give rise to the experimentally observed low detection limits for Pb2+. These PCCA materials enable stimulated hydrogel volume transitions to be studied.

Acetylcholinesterase-Based Organophosphate Nerve Agent Sensing Photonic Crystal

We developed a polymerized crystalline colloidal array (PCCA) photonic crystal sensing material that senses the organophosphorus compound parathion at ultratrace concentrations in aqueous solutions. A periodic array of colloidal particles is embedded in a hydrogel network with a lattice spacing such that it Bragg diffracts visible light. The molecular recognition agent for the sensor is the enzyme acetylcholinesterase (AChE), which binds organophosphorus compounds irreversibly, creating an anionic phosphonyl species. This charged species creates a Donnan potential, which swells the hydrogel network, which increases the embedded particle array lattice spacing and causes a red-shift in the wavelength of light diffracted. The magnitude of the diffraction red-shift is proportional to the amount of bound parathion. These AChE-PCCAs act as dosimeters for parathion since it irreversibly binds. Parathion concentrations as low as 4.26 fM are easily detected.

Noninvasive Photonic-Crystal Material for Sensing Glucose in Tears

Most noninvasive (NI) methods for the determination of glucose either detect a small specific glucose signal or measure the effect of glucose on a tissue optical property (1)(2). A recent review identified the three main issues in NI glucose measurements as specificity, compartmentalization of glucose values, and calibration (1). This editorial discusses a photonic crystal method (3), with respect to these issues. Asher’s group have developed a novel photonic sensing material that responds to glucose concentrations via diffraction of visible light. Polymerized crystalline colloidal arrays (PCCAs) are periodic crystalline lattices of polystyrene microspheres polymerized within thin hydrogel films (3)(4)(5)(6)(7). The arrays are brightly colored and act as diffraction gratings for white light according to the Bragg diffraction equation (4)(5): \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[{\Delta}{\lambda} {=} \frac{n {\times} d}{\mathrm{sin}{\Theta}}\] \end{document} In Eq. 1, n is the refractive index of the system (medium, hydrogel, and colloids), d is the spacing between the diffracting planes, λ is the diffracted wavelength, and Θ is the glancing angle between the incident light and the diffracting planes. A change in electric charge in the PCCAs resulting from binding of molecular or ionic species causes changes the spacing, d , and there is a subsequent wavelength shift, Δλ, of the light reflected off the array. Asher’s group have constructed a photonic glucose sensor in the form of thin acrylamide PCCA hydrogel films that contain glucose molecular recognition elements (3)(6)(7). Phenylboronic acid derivatives …

Electrochemical Investigation of Pb2+ Binding and Transport through a Polymerized Crystalline Colloidal Array Hydrogel Containing Benzo-18-crown-6

The transport of Pb2+ through a sensory gel, a polymerized crystalline colloidal array hydrogel with immobilized benzo-18-crown-6, is important for understanding and optimizing the sensor. Square wave voltammetry at a Hg/Au electrode reveals many parameters. The partition coefficient for Pb2+ into a control gel (no crown ether), K(p), is 1.00 +/- 0.018 (errors reported are SEM). The porosity, epsilon, of the gel is 0.90 +/- 0.01. Log K(c) for complexation in the gel is 2.75 +/- 0.014. Log K(c) in aqueous solution for Pb2+ with the ligand 4-acryloylamidobenzo-18-crown-6 is 3.01 +/- 0.010 with dissociation rate k(d) = (8.34 +/- 0.45) x 10(2) s(-1) and association rate k(f) = (8.79 +/- 0.025) x 10(7) M(-1) s(-1). The partition coefficient of the ligand 4-acryloylamidobenzo-18-crown-6 into the control gel, K(p,L) is 2.07 +/- 0.15. The diffusion coefficient of Pb2+ in the control gel is 6.72 x 10(-6) +/- 0.12 cm(2)/s. For the sensor gel, but not control gel, diffusion coefficients are location dependent. The range of diffusion coefficients for Pb2+ in the probed locations was found to be (6.11-12.60) x 10(-7) cm(2)/s for 0.91 mM Pb2+ and (2.84-9.39) x 10(-7) cm(2)/s for 0.35 mM Pb2+. Lead binding in the sensor gel is slightly less avid than in solution. This is attributed, in part, to the demonstrated affinity of the ligand 4-acryloylamidobenzo-18-crown-6 to the gel. Diffusion coefficients determined for the sensor gel were found to be location dependent. This is attributed to heterogeneities in the crown concentration in the gel. Analysis of diffusion coefficients and rate constants show that diffusion and not chemical relaxation will limit the time response of the material.

Epoxide functionalized polymerized crystalline colloidal arrays

We report the development of a novel method for functionalizing polymerized crystalline colloidal arrays (PCCA). This new method enables us to easily incorporate molecular recognition agents into the PCCA to produce chemical sensing materials that report on analyte concentration via diffraction of visible light. We copolymerize glycidyl methacrylate with acrylamide and N,N′-methylenebisacrylamide around crystalline colloidal arrays. The incorporated epoxide groups are available for further PCCA functionalization through reactions with thiol, amine or hydroxyl groups. We fabricated a Pb 2+ sensor, which utilizes a crown ether recognition agent, as well as a glucose sensor, which utilizes glucose oxidase.

A general photonic crystal sensing motif: creatinine in bodily fluids.

We developed a new sensing motif for the detection and quantification of creatinine, which is an important small molecule marker of renal dysfunction. This novel sensor motif is based on our intelligent polymerized crystalline colloidal array (IPCCA) materials, in which a three-dimensional crystalline colloidal array (CCA) of monodisperse, highly charged polystyrene latex particles are polymerized within lightly cross-linked polyacrylamide hydrogels. These composite hydrogels are photonic crystals in which the embedded CCA diffracts visible light and appears intensely colored. Volume phase transitions of the hydrogel cause changes in the CCA lattice spacings which change the diffracted wavelength of light. We functionalized the hydrogel with two coupled recognition modules, a creatinine deiminase (CD) enzyme and a 2-nitrophenol (2NPh) titrating group. Creatinine within the gel is rapidly hydrolyzed by the CD enzyme in a reaction which releases OH(-). This elevates the steady-state pH within the hydrogel as compared to the exterior solution. In response, the 2NPh is deprotonated. The increased solubility of the phenolate species as compared to that of the neutral phenols causes a hydrogel swelling which red-shifts the IPCCA diffraction. This photonic crystal IPCCA senses physiologically relevant creatinine levels, with a detection limit of 6 microM, at physiological pH and salinity. This sensor also determines physiological levels of creatinine in human blood serum samples. This sensing technology platform is quite general. It may be used to fabricate photonic crystal sensors for any species for which there exists an enzyme which catalyzes it to release H(+) or OH(-).

Photonic crystal aqueous metal cation sensing materials.

We developed a polymerized crystalline colloidal array photonic material that senses metal cations in water at low concentrations (PCCACS). Metal cations such as Cu2+, Co2+, Ni2+, and Zn2+ bind to 8-hydroxyquinoline groups covalently attached to the PCCACS. At low metal concentrations (<microM), the cations form bisliganded complexes with two 8-hydroxyquinolines that cross-link the hydrogel and cause it to shrink, which blue shifts the photonic crystal diffraction. These bisliganded cross-links break at higher cation concentrations due to the formation of monoliganded cation complexes. This red shifts the diffraction. We have extended hydrogel volume phase transition theory in order to quantitatively model the diffraction dependence upon metal concentration. These materials can be used as a dosimeter to sense extremely low metal cation concentrations or as a sensor material for concentrations greater than 1 microM. Metal cation concentrations can be determined visually from the color of the diffracted light or can be determined by reflectance measurements using a spectrophotometer. This sensing material could be used in the field to visually determine metal cation concentrations in drinking water. A color chart would be used to relate the diffracted color to the metal cation concentration.

Polymerized crystalline colloidal array chemical-sensing materials for detection of lead in body fluids.

We have developed intelligent polymerized crystalline colloidal array (IPCCA) chemical-sensing materials for detection of Pb(2+) in high ionic-strength environments such as body fluids with a detection limit of <500 nmol L(-1) Pb(2+) (100 ppb). This IPCCA lead sensor consists of a mesoscopically periodic array of colloidal particles polymerized into an acrylamide hydrogel. The array Bragg-diffracts light in the visible spectral region because of the periodic spacing of the colloidal particles. This material also contains a crown ether chelating agent for Pb(2+). Chelation of Pb(2+) by the IPCCA in low-ionic-strength solutions results in a Donnan potential that swells the gel, which red-shifts the diffracted light in proportion to the Pb(2+) concentration. At high ionic strength the Donnan potential is, unfortunately, swamped and no static response occurs for these sensors. We demonstrate, however, that we can determine Pb(2+) at high ionic strength by incubating these IPCCA in a sample solution and then measuring their transient response on exposure to pure water. The non-complexed ions diffuse from the IPCCA faster than the bound Pb(2+). The resulting transient IPCCA diffraction red-shift is proportional to the concentration of Pb(2+) in the sample. These IPCCA sensors can thus be used as sensing materials in optrodes to determine Pb(2+) in high-ionic-strength solutions such as body fluids.

Development of an intelligent polymerized crystalline colloidal array colorimetric reagent.

We have developed a novel colorimetric reagent for the determination of Pb2+, pH, and temperature. This colorimetric reagent consists of a dispersion of approximately 100-microm particles composed of an intelligent polymerized crystalline colloidal array (IPCCA). The IPCCA particles are composed of a hydrogel polymerized around a face-centered cubic (fcc) array of monodisperse, highly charged polystyrene colloidal particles. These IPCCA particles diffract visible light because the (111) planes of the fcc polystyrene colloidal particle array have an approximately 200-nm lattice constant. The IPCCA particles also contain a molecular recognition agent that actuates array volume changes as a result of changes in analyte concentration or temperature. This results in changes in the IPCCA lattice constants, which shifts the wavelength of light diffracted. We report here the use of these sensing materials in a liquid dispersion that can be poured into a sample solution. This diffraction measurement method is analogous to X-ray powder diffraction measurements. The diffraction wavelength is monitored at a defined angle relative to the incident light.

Photonic Crystal Chemical Sensors: pH and Ionic Strength

Diffraction from a photonic crystal material composed of a hydrolyzed polymerized crystalline colloidal array (PCCA) can be used to sense pH and ionic strength. The PCCA is a polyacrylamide hydrogel which embeds a polystyrene crystalline colloidal array (CCA). The diffracted wavelength of the PCCA changes as the PCCA volume changes due to the alterations in the CCA lattice constant. We examine the pH and ionic strength dependence of the hydrolyzed PCCA volume by monitoring the Bragg diffracted wavelength. We also develop a zero free parameter quantitative model to describe the pH and ionic strength dependence of the hydrogel volume.

Synthesis of Highly Fluorinated Monodisperse Colloids for Low Refractive Index Crystalline Colloidal Arrays

Highly charged fluorinated monodisperse spherical particles of diameters between 50 and 250 nm were synthesized from 1H,1H-heptafluorobutyl methacrylate by emulsion polymerization. These particles have a low refractive index of 1.386. High particle surface charge densities were obtained by minimizing the polymer molecular weight. These colloids formed well-ordered crystalline colloidal arrays (CCAs), after dialysis and ion exchange, which Bragg diffract light at wavelengths from the near-IR down to 270 nm in the UV. The diffraction bandwidths in water are very narrow (<10 nm) due to the closeness of refractive index of the colloidal particles to that of water. These CCAs are excellent materials for narrow band notch optical filters. In addition, these fluorinated CCAs can be easily refractive index matched to a predominately aqueous medium. We have covalently attached dyes to the colloidal particles to prepare absorbing CCAs. We photopolymerized these dyed CCAs within a polyacrylamide matrix to form polymerized crystalline colloidal array (PCCA). These semisolid PCCAs can withstand vibrations, ionic impurity addition, and thermal shocks while maintaining the CCA ordering. The medium within the PCCA can easily be exchanged to exactly refractive index match the CCA. These refractive index matched dyed PCCAs may have applications in optical limiting, computing, and nanosecond fast optical switching devices as discussed in the accompanying paper.

Intelligent Polymerized Crystalline Colloidal Arrays: Novel Chemical Sensor Materials

We report the development of a novel sensing material that reports on analyte concentrations via diffraction of visible light from a polymerized crystalline colloidal array (PCCA). The PCCA is a mesoscopically periodic crystalline colloidal array (CCA) of spherical polystyrene colloids polymerized within a thin, intelligent polymer hydrogel film. CCAs are brightly colored, and they efficiently diffract visible light meeting the Bragg condition. The intelligent hydrogel incorporates chemical molecular recognition agents that cause the gel to swell in response to the concentration of particular analytes; the gel volume is a function of the analyte concentration. The color diffracted from the hydrogel film is, thus, a function of analyte concentration: the swelling of the gel changes the periodicity of the CCA, which results in a shift in the diffracted wavelength. We have fabricated a sensor, utilizing a crown ether as the recognition agent, that detects Pb2+ in the 0.1 μM−20mM (∼20 ppb−∼4000 ppm) concenra...