Optical Sensors for the Detection of Trace Chloroform Jonathan K. Fong, † Justin K. Pena, † Zi-Ling Xue,* ,† Maksudul M. Alam, ‡ Uma Sampathkumaran, ‡ and Kisholoy Goswami ‡ † Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600, United States ‡ InnoSense LLC, 2531 West 237th Street, Suite 127, Torrance, California 90505-5245, United States *S Supporting Information ABSTRACT: Optical thin film sensors have been developed to detect chloroform in aqueous and nonaqueous solutions. These sensors utilize a modified Fujiwara reaction, one of the only known methods for detecting halogenated hydrocarbons in the visible spectrum. The modified Fujiwara reagents, 2,2′-dipyridyl and tetra-n-butyl ammonium hydroxide (n- Bu 4 NOH or TBAH), are encapsulated in an ethyl cellulose (EC) or sol−gel film. Upon exposure of the EC sensor film to HCCl 3 in petroleum ether, a colored product is produced within the film, which is analyzed spectroscopically, yielding a detection limit of 0.830 ppm (parts per million v/v or μL/L hereinafter) and a quantification limit of 2.77 ppm. When the chloroform concentration in pentane is ≥5 ppm, the color change of the EC sensor is visible to the naked eye. In aqueous chloroform solution, reaction in the sol−gel sensor film turns the sensor from colorless to dark yellow/brown, also visible to the naked eye, with a detection limit of 500 ppm. This is well below the solubility of chloroform in water (ca. 5,800 ppm). To our knowledge, these are the first optical quality thin film sensors using Fujiwara reactions for halogenated hydrocarbon detection. C hlorinated hydrocarbons (CHCs) pose a serious threat to the environment. 1−10 CHCs such as chloroform (HCCl 3 ) 1 are widely used in many industries as a solvent, refrigerant, and pesticide. 2,3 The extensive use and production of HCCl 3 is on the scale of thousands of tons per year worldwide, which results in increased disposal of HCCl 3 into the environment in the form of aqueous wastes. One of the greatest concerns of CHCs such as HCCl 3 is that, with a solubility of 8.7(0.5) g/L or [5.8(0.3) × 10 3 μL/L or ca. 5,800 ppm] in water at 23−24 °C, 4 it can easily cause the pollution of groundwater, soils, and sediments. 2−6 With their density higher than that of water, chloroform and other similar pollutants, known as dense nonaqueous phase liquids (DNAPLs), can move through subsurface soils and groundwater, forming DNAPL pools. 2,3,5 Both the DNAPL soil residuals and the pools are slowly dissolving sources of groundwater and soil contamination, at levels as high as, e.g., ca. 689 ppm (or 1.02 g/ L) of HCCl 3 in the core of the dissolved plume in a sandy aquifer. 5 Generally, CHCs in low concentrations in air or in water can cause damage to the liver, kidneys, and central nervous system. Most CHCs, including HCCl 3 , are also suspected carcinogens. 11−13 Moreover, it has been shown that organic solvents, such as HCCl 3 used in the manufacturing processes for drug products, are often not completely eliminated. 14−17 Thus, low levels of residual organic solvents are present in most pharmaceutical products. The acceptable level of HCCl 3 at 60 ppm in pharmaceuticals is given in guidelines issued by the International Conference on Harmonization (ICH). 14−17 Therefore, monitoring the con- centration of HCCl 3 in groundwater and quality control of residual HCCl 3 in pharmaceutical samples is vital for the environment and the pharmaceutical industry. 17,18 Several common methods have been established for HCCl 3 detection, including GC-MS, usually in tandem with preconcentration, such as purge and trap, solid-phase micro- extraction (SPME), and headspace/SPME analysis. 19−25 Infra- red spectroscopy has also been studied for CHC detection in water. 26−28 While these methods provide sensitive and adequate detection limits, they often require relatively expensive, nonportable equipment and trained technicians. In addition, they are time-consuming and difficult to apply for on- site monitoring. Development of sensors and new methods for chemical detection is an active area of research. For example, nano-based chemiluminescence sensor arrays and surfaced- grafted quantum dots have been reported for the detection of other chemicals. 29,30 There have been drives to develop a simple, direct, and low cost method to analyze HCCl 3 as an environmental pollutant as well as to determine residual concentration in pharmaceutical samples. Detection of CHCs in the visible range is generally based on Fujiwara reactions in solution. 31−36 The Fujiwara reaction for the spectroscopic detection of HCCl 3 was first reported in 1916 and originally relied on a two phase system consisting of an aqueous layer of NaOH along with a liquid pyridine layer to which HCCl 3 was added. 37 This mixture was then heated to Received: July 9, 2014 Accepted: December 31, 2014 Published: December 31, 2014 Article pubs.acs.org/ac © 2014 American Chemical Society 1569 DOI: 10.1021/ac503920c Anal. Chem. 2015, 87, 1569−1574