Test Strips for Heavy‐Metal Ions Fabricated from Nanosized Dye Compounds

Abstract

Analytical test strips for heavy-metal ions can provide simple and convenient procedures for on-site analysis and daily monitoring of water quality without using costly instruments. However, since the standard limit values of heavy metals in the environment and drinking water are remarkably low (ppb level), the detection limits of commercial test strips (ppm level) are insufficient to satisfy the required criteria. In addition, since they are generally prepared by soaking paper in the signaling reagent, leakage of reagent is facile, and hence reliability and sensitivity of detection are poor. Firm fixation of colorimetric and fluorometric reagents for metals on solid substrates has been attempted by incorporation into PVCbased liquid membranes, [1] covalent anchoring with crosslinked copolymers, [2] and layer-by-layer accumulation methods such as Langmuir-Blodgett films [3,4] and alternate deposition of oppositely charged polyelectrolytes. [5,6] A wealth of information on optical sensors and probes for simple determination of heavy metals has been reviewed. [7] However, drawbacks include complicated synthetic procedures, insufficient sensitivity, requirements for auxiliary additives, and difficulty in controlling the concentration of the reagent and its uniformity. Hence, we sought a simple procedure for firm and uniform coating of signaling reagents on an appropriate substrate without any additional matrix.Recently, a simple and versatile process known as the "reprecipitation method" was proposed for the preparation of aqueous dispersions of organic nanoparticles.[8] The process involves injection of a water-miscible organic solution into water with vigorous stirring. We found that various organic nanoparticles can be uniformly coated onto a membrane filter (cellulose ester type) to form a thin layer simply by filtration of the nanoparticle dispersion. We attempted to apply this simple procedure to the fabrication of analytical test strips from a selection of signaling reagents. We examined popular indicator dyes having an aromatic conjugated system: dithizone (Dith), 1-(2-thiazolylazo)-2-naphthol (TAN), 1-(2-pyridylazo)-2-naphthol (PAN), tetraphenylporphin (TPP), and bathophenanthroline (Bathophen).Optimized preparative conditions for aqueous dispersions of the indicator dyes are listed in Table 1. Water-miscible acetone and THF were used for dissolution of the dyes. The pH of the aqueous phase was controlled to maintain electroneutrality of the dye species by considering the pK a values of the respective compounds. Smaller particles are formed with increasing temperature of the aqueous phase and with decreasing dye concentration in the organic phase.[9] Figure 1 shows scanning electron microscopy (SEM) images of TPP nanoparticles and PAN nanofibers. Round particles (50-100 nm) were mainly formed from TPP and Bathophen. In contrast, fibrous products were formed in the case of PAN, TAN, and Dith. These dyes commonly have dissociative protons capable of forming intermolecular hydrogen bonds which assist formation...

Dye Nanoparticle or Nanocomposite-Coated Test Papers for Detection at Ppb Levels of Harmful Ions

“…8) [12]. Immersion test is just the same detection way as using any ordinary test papers such as urine, pH, ions etc., but it takes longer to complete sufficient color development with DNTS than those with other test papers well known as dip and read test.…”

“…When the dispersion was left for a long time, overgrowth of crystals induced large aggregates, resulting in nonuniformity and low surface area of the dithizone fiber layer. Figure 4 shows scanning electron microscopy (SEM) images of TPP nanoparticles and PAN nanofibers [12]. Round particles (50-100 nm) were mainly formed from TPP (tetraphenylporphyrin) and bathophen (bathophenanthroline).…”

“…Herein we have reported detection membranes loaded with dye nanoparticles and succeeded the detection at ppb level, named "dye nanoparticle-coated test strip" (DNTS) [12]. The fabrication method is very simple and feasible but applicable to a wide variety of indicator dyes by using two preparation processes of nano-dispersion.…”

A fluorometric paper-based sensor array for the discrimination of heavy-metal ions

“…As a result, there is an urgent need for lowcost, highly sensitive and selective methods of discrimination of heavy-metal ions, especially at their wastewater discharge standard concentrations (some of them are as low as 10 À 7 M). Instead of trivial analytical instrument methods, a number of inexpensive colorimetric sensors for the detection of heavy-metal ions have been proposed (e.g., modified or functionalized nanoparticles, DNAzyme, nanosized dye based test strips, etc) [10][11][12]. Combining with sensor array technology and pattern recognition method, optical sensor arrays have been extensively applied for the determination of toxic industrial gases [13,14], nucleic acid and biomolecules [15], diols [16], and even for diagnostic [17], etc.…”

Test Strips for Lead(II) Based on a Unique Color Change of PVC-film Containing O-Donor Macrocycles and an Anionic Dye

“…Recently, we have developed highly sensitive detection systems of a thin solid phase, such as a nanofibrous membrane filter for lead(II) 1 and test strips immobilized with nanosize dyes. 2 They allow the simple detection of analyte by the naked eye, or by portable spectrophotometers without any specific pretreatment. Optical film sensors known as "optode", have been proposed to be applied to environmental monitoring.…”