Asher Research Group

Department of Chemistry University of Pittsburgh

 

Colloid Group

 

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Diffraction:

 

You may be familiar with the phenomena of diffraction already and not even be aware of it. You see forms of diffraction in many everyday objects (see pictures below). We, in the Asher Research Group, exploit this natural phenomena for developing novel materials for sensing and other applications.

 

 

Bragg diffraction was first proposed by William Lawrence Bragg in 1912 as a means of analyzing the structure of crystals. Bragg and his father (William Henry Bragg) collimated x-rays to diffract off of different crystal planes. The x-rays were then collected in an ionization chamber and the level of ionization was measured as a function of the incident angle of the x-rays. Using this method, the Braggs were able to determine the crystalline spacing for a number of substances.

The Bragg Condition is given by:

 

 

Waves that satisfy this condition interfere constructively and have the same apparent strength as reflection.

 

 

Photonic crystals are periodic dielectric or metallo-dielectric (nano)structures that are designed to affect the propagation of electromagnetic waves. Since the basic physical phenomenon is based on diffraction, the periodicity of the photonic crystal structure has to be in the same length scale as the wavelength of the electromagnetic waves (~300 nm for photonic crystals operating in the visible part of the spectrum). Photonic crystals are attractive optical materials for controlling and manipulating the flow of light. They are of great interest both for fundamental and applied research.

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Crystalline Colloidal Array Fabrication:

 

CCAs are typically synthesized through a free-radical heterogeneous nucleation emulsion polymerization. This synthesis can be used to prepare particles that are 100 - 400 NM in diameter. The reactants for the polymerization include an emulsifier, slightly water-soluble monomers, a less water-soluble crosslinker, a water-soluble initiator, and a buffer in an aqueous polymerization medium. The surface charge and size of the resulting particles can be altered by varying the relative amounts of these reactants.

 

 

 

Aqueous suspensions of monodisperse, highly-charged polystyrene particles self-assemble into highly-ordered three-dimensional arrays, known as crystalline colloidal arrays (CCAs). This self-assembling occurs in low ionic strength media due to electrostatic repulsion, originating from the ionization of sulfonate and carboxylate groups on the surface of each sphere. The system minimizes its free energy by assembling into either a face-centered cubic (FCC) or body-centered cubic (BCC) lattice.

 

 

The periodic modulation in the CCA refractive index and the lattice spacing of the array are such that visible light is diffracted according to Bragg's Law. The lattice spacing can be tuned, either by changing the particle size or the particle concentration, so that CCAs can efficiently diffract light in the near-UV, visible and near-IR spectral regions. .

 

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Polymerized Crystalline Colloidal Array Fabrication:

 

Since the ordering of the CCA depends on the electrostatic repulsion between particles, the lattice will disorder in the presence of ionic impurities. The CCA lattice can be stabilized by polymerizing it within a hydrogel to form a polymerized crystalline colloidal array (PCCA). To form the PCCA, nonionic polymerizable monomers, cross-linkers and photoinitiators are dissolved into the CCA. The mixture is then polymerized within a quartz cell.

 

 

 

The resulting PCCA is considerably more robust than the CCA. Also, since the CCA lattice is embedded within the hydrogel, the observed diffraction closely follows the hydrogel volume. As the hydrogel undergoes a volume change, the spacing between the CCA particles will change, resulting in a change in observed diffraction. To apply this motif to chemical sensing, molecular recognition groups are incorporated into the hydrogel backbone either during or after polymerization. The resulting intelligent polymerized crystalline colloidal array (IPCCA) will optically report on the concentration of the analyte of interest. As the analyte of interest is bound, there will be a change in the free energy of the system, resulting in a change in the equilibrium volume of the hydrogel and a change in the observed diffraction from the PCCA.

 

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Some Group Projects:

Development of 2-Dimensional PCCA
Study of Interactions between Nanoparticles within CCA
Method Development for Measurement of Glucose Concentrations in Tear Fluid
Physiological pH Sensing Material
Characterization of Mechanical Properties of Nanocomposite PCCA Materials
Inverted-Structure Crystalline Material
more to come soon

 

Glucose Sensing Material

 

  • We are developing a photonic crystal glucose sensor consisting of a crystalline colloidal array embedded within a polymer network with pendent phenylboronic acid groups. The pendent boronic acid groups bind glucose in a "sandwich-like" complex, forming additional crosslinks in the hydrogel. As these additional crosslinks form, the hydrogel shrinks and the diffraction blue-shifts in proportion to the glucose in solution (see picture below).
  • We are optimizing our photonic glucose sensing motif to sense glucose at the low glucose concentrations of tear fluid. This sensing motif will be utilized for the fabrication of noninvasive or minimally invasive in vivo glucose sensing materials in the form of ocular inserts or diagnostic contact lenses for patients with diabetes mellitus (see second picture below).
  • We are also developing the material to sense glucose at higher concentrations, such as found in blood. Successful sensor development has direct application in monitoring glucose levels of inpatients in acute care situations, both those with diagnosed diabetes mellitus and those who experience acute care induced hyperglycemia.
  • This technology has been licensed for development by Glucose Sensing Technologies, LLC.

 

 

 

Mass media coverage:

 

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Cancer Protein Marker Sensing Material

 

  • There is intense interest in developing in vivo sensors for clinically important analytes and for markers of disease. We report here progress in a program which seeks to create a new sensor material which will be implanted under the skin to sample interstitial fluid.
  • This material will be used to detect protein markers of cancer.
  • Our sensor material consists of a crystalline colloidal array hydrogel photonic crystal which will contain antibodies for these protein markers. The signaling response utilizes the Bragg diffraction of light by an embedded face centered cubic (fcc) array of colloidal particles.

 

 

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Organophosphate Sensing Material

 

  • We have developed an intelligent PCCA photonic crystal sensing material which can sense organophosphate compounds at ultra-trace concentrations in aqueous solutions.
  • A periodic array of colloidal particles is embedded in an acrylamide polymer hydrogel network, with a lattice spacing such that it Bragg diffracts visible light.
  • The molecular recognition agent for the sensor is the enzyme acetylcholinesterase, which binds organophosphates irreversibly, creating an anionic phosphonyl species.
  • The charged species creates a Donnan potential between the interior and exterior of the hydrogel, which causes an osmotic pressure in the hydrogel network, increasing the gel volume . The volume change increases the lattice spacing of the embedded CCA, causing a red-shift in the wavelength of light diffracted.
  • These AChE-PCCAs act as dosimeters, as they irreversibly bind all parathion in solution below stoichiometric ratios. The degree of redshift in diffraction observed is proportional to the amount of bound organophosphate. Parathion concentrations as low as 4.26 fM are easily detected.
  • This sensor can be used to determine ultra-trace concentrations of organophosphates present in groundwater sources and soils. In addition, it can be utilized with an air aspiration mechanism to detect air-borne chemical warfare agents.

 

 

 

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Point-of-Care Ammonia Sensing Material

 

  • Ammonia arises from deamination of amino acids. Hyperammonemia occurs in it least four groups of inborn errors of metabolism: urea cycle disorders, organic acidemias, fatty acid oxidation defects, and liver malfunction. A point of care sensor capable of determining ammonia concentration from 10 to 300 ÁM is valuable for diagnosis and management of these patients.
  • We are developing point-of-care test kit sensors for determining ammonia. The sensors would also be applied to measure amino acid levels through working with different deamination enzymes and subsequently sensing ammonia.
  • An ammonia sensor, which utilizes the well-known Berthelot reaction, has been developed in our group. Ammonia blue-shifts the PCCA by forming crosslinks with two phenol groups tethered to the hydrogel backbone.
  • We have currently coupled glutamate dehydrogenase (GLDH) onto a pH sensitive PCCA in the development of a second generation ammonia sensor. The immobilized enzyme catalyzed reaction consumes H+, which results in a red-shift of diffraction. We are working on improving the sensor response to ammonia concentration under physiological conditions.

 

 

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Zinc Sulfide Particle Synthesis

 

  • We have synthesized monodisperse ZnS particles of various sizes between 100 NM and 500 NM and coated them with polyelectrolyte molecules.
  • These core-shell particles are highly charged and self-assemble into a stable crystalline colloidal array.
  • This CCA demonstrates Bragg diffraction peaks with unusually strong intensity and wide band width, owing to the high index of refraction of ZnS (~2).
  • It can be used in applications such as coatings, filters and photonic crystal sensors and devices.

 

 

 

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