Because of their catalytic versatility, metals, such as Fe, Mg, Ca and Cu, have been adopted by biological systems, but their reactivity can also make them toxic at low concentrations and classified as poisons.  Current means of heavy metal detection has become an increasingly complex process and the sophisticated laboratory analysis of trace levels of heavy metal ions remains unmatched for selectivity and precision.  An on-site, over-the-counter detector for heavy metals would simplify the detection and provide a quick and useful means for monitoring contaminant levels in household and remote well waters. Structurally colored thin films offer a promising materials platform for the development of a metal ion sensor due to the rapid, straightforward, field-readable response of the visual color change which occurs upon exposure to metal ions.  Our research has focused on cross-linked thin films of chitosan, thiol modified chitosan, and alginate.

Chitin is a colorless biopolymer found in shells of crabs and shrimp and is the second most abundant poly(saccharide) found in nature.  Chitosan, the acid soluble form of chitin, is a non-toxic, biodegradable, biopolymer consisting primarily of β(1→4) linked 2-amino-2-deoxy-β-D-glucopyranose units, and is currently used in tissue engineering, antifouling coatings, separation membranes, stent coatings, enzyme immobilization matrices, and as a platform for a taste sensor, optical devices and removal of heavy metals from ground and wastewater.

Chitosan is a commercially interesting compound because of its high nitrogen content (6.89%), making chitosan a useful chelating agent for metal ions.  Many factors determine chitosan’s capacity for absorption of metal ions including pH, concentration, temperature, percent deacetylation, interaction time, and chain length.  For example, if the pH is below the pKa (between 6.2-6.8) of chitosan, then the amines become approximately 90% protonated, resulting in a positively charged chitosan, which is a good adsorbent for anions.  

Because chitosan is well known for its metal ion absorbance capabilities, interference-colored, thin single films of chitosan were investigated for their ability to distinguish metal ions using a change in optical properties and thickness.  After coating chitosan- hexamethylene 1,6-di(aminocarboxysulfonate) (HDACS) solution onto a polished silicon wafer, the films are measured using reflectance and ellipsometry before and after dipping into a 50 ppm (parts per million) aqueous metal salt solution.  Chitosan-HDACS films increased in thickness and red shifted their reflectance spectra in the presence of CrO₃.

 Investigations into the modification of chitosan by the addition of thiol groups and their subsequent processing will be discussed. Basically, increasing the amount of thiol added to the chitosan backbone can correlate with an increase in sensitivity, especially for mercury nitrate.  Investigation of the 6.67% S thiol glycolic chitosan indicates that the films only red shifted when interacting with the mercury nitrate solution. All other solutions blue shifted, creating a specific sensor for mercury.

Alginates are a family of unbranched binary copolymers of (1à4)-linked b-D-mannuronic acid (M) and a-L-guluronic acid (G) groups.  Alginates with various M and G compositions are produced by a number of organisms, and are constituents in the cell walls of brown algae.  To demonstrate the repeatability of the spin coating deposition process, twenty-four films were spun from a single solution using identical deposition parameters.  Each film was exposed to aerosolized CaCl₂ solution for ten seconds before being rinsed and dried.  This aerosol application technique allows the calcium to be evenly applied over the film’s surface without the problems associated with directly dipping the films into solution.  The evenness of cross-linking is evidenced by the films’ retained smoothness and virtually constant radial coloration from center to edge. Ellipsometry and reflectance data were gathered for each sample.  Film thicknesses and indices of refraction were determined using an M-2000U variable-angle spectroscopic ellipsometer. The Ca²⁺-alginate films were sensitive to lead and chromium (VI and II).

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