Detection of Metal Ions

Functionalized ICP chains can be also used for the detection of non-electroactive alkaline and alkaline earth metals by exploiting quite a different approach the coordination of the ICP molecules with the metal ions affects the physico-chemical properties of the polymer film, inducing a strong variation of the relevant voltammetric trace. In particular, the p-doping process results conditioned by the formation of more or less strong chemical interactions between the metal analyte 

---

The Application of Electron Transfer Process Toward Visual Detection of Metal Ions... 

F.J. Hayes, H.B. Halsall, and W.R. Heineman, Simultaneous immunoassay using electrochemical detection of metal ion labels. Anal. Chem. 66, 1860-1865 (1994). 

In analytical chemistry, detection of metal ions is of major importance. In particular, the development of simple and reliable methods for continuous control in situ of metal ions in the environment is the object of much attention. For instance, the detection of lead, mercury, cadmium, and iron ions in sea water will be performed in the near future by optodes associated with suitable fluoroionophores, thus allowing continuous monitoring by instruments on ships. 

The intent of this chapter is not to provide an exhaustive review of chemical- and biosensors and probes, but rather to offer a brief overview of existing optical techniques and an indepth analysis of near-infrared (NIR) fluorogenic probes and sensors for the detection of metal ions, solution pH, and biomolecules and to present some of the latest results. 

Poly(2-hydroxyethyl methylmethacrylate) (PHEMA) has been used as a matrix for the detection of metal ions. 79 A near-IR dye (2,3-naphthalocyanine-tetrasulphonic acid) was immobilized in a polymer matrix which was attached to the reaction phase of two optical fibers. A mixture of the matrix and the dye was prepared by mixing PHEMA and dye in a 60/40 ratio. The optimum ratio of polymer and dye were not fully investigated. The dye/polymer mixture was applied to the tip of the probe in 10-to 15-/iL aliquots forming a thin coating on the probe after solvent evaporation as shown in Figure 7.9. 

An NIR optical fiber for the detection of metal ions has been developed In a controlled environment, the optical probe with immobilized NIR dye was immersed in vials containing different metal ions varying in concentrations from 10 9 to 1CT2 M. The probe response was obtained by the diffusion of the metal ions through the poly(2-hydroxyethyl methacrylate) (PHEMA) polymer matrix where the dye was covalently attached. On complexation of the metal with the dye, the intensity of the fluorescence signal increased. 

The use of chemiluminescence reactions for the detection of metal ions by liquid chromatography was recently reported [59,60]. The detectors made use of the chemiluminescence produced in the reaction between luminol and hydrogen peroxide which is catalyzed by transition metals. The column effluent was mixed with the reagents in order to yield the chemiluminescence. The reaction was fast and was carried out at room temperature. By varying the pH of the buffer, selectivity towards certain metals was also achieved. For example, at pH 10-11 nickel could be analyzed but lead and aluminium were inactive at pH 13-14, the converse was true [59]. Aminco-Bowman has marketed a liquid chromatographic system in which amino acids and amines are analyzed by means of the fluorescence produced on reaction with the reagent fluorescamine. Fluorescamine does not fluoresce, but it does react with primary amino groups to produce fluorescent derivatives. The reaction is instantaneous and may be carried out at room temperature, usually at pH 9. This detection system promises to be far more sensitive than the ninhydrin detection system and is much more easily adapted to HPLC. 

The generic approach to modifying electrodes with peptides for the detection of metal ions is depicted in Fig. 10.1 and outlined in Procedure 13 (in CD accompanying this book). The essential feature is an electrode modified with an SAM, which contains a carboxylic acid moiety at the distal end. The carboxylic acid moiety is activated using carbodiimides, typically l-ethyl-3-(3-dimethylaminopropyl)... 

The electrochemical approaches developed in the aforementioned studies are based on detection of metal ions released from the sandwich. The use of metallic gold as a source of electrochemical data is described in our earlier paper [30], but the stage of metal releasing is omitted in the work mentioned. The present study also deals with the use of a metal label as the signal generating substance. 

J. Tanyanyiwa and P.C. Hauser, High-voltage contactless conductivity detection of metal ions in capillary electrophoresis, Electrophoresis, 23 (2002) 3781-3786. 

The technical difficulties of making films which, both work and stay in place are immense. But now the breakthrough has happened, many other and more complex chemistries have been fixed on film. Film is available for urea, triglycerides and anylase in blood sera, and even ion selective electrodes have been adapted for the detection of metal ions such as Na +, K +, etc. in a single step. 

This effect has been successfully employed to improve the LC detection of metal ions as their metal complexes (496.497.499). Recently, it has also been demonstrated that metal ions can be detected by direct-current argon plasma emission spectroscopy after LC separation with micellar mobile phases (490). 

More importantly, the use of heavy metal anionic micellar media has been shown to allow for observation of analytically useful room-temperature liquid phosphorescence (RTLP) (7.484.487). There are several examples in which phosphorescence has been employed as a LC detector with the required micellar assembly being present as part of the LC mobile phase (482) or added post column (485). More recently, metal ions have been determined in a coacervate scum by utilizing the micellar-stabilized RTLP approach (498). Thus, the future should see further development in RTLP detection of metal ions in separation science applications. 

A keen recent interest in polyimine ruthenium(II) complexes (tris-bipyridinates, tris-phenanthrolinates, and their analogues) has largely been evoked by the ample scope they offer as selective DNA-cleaving agents and probes in biochemistry. Such ruthenium(II) complexes, as well as their photophysics, are of particular interest in creating the devices for molecular electronics (e.g., systems of the light-switch type) and in analytical detection of metal ions as well. 

Use of apoenzymes for the detection of metal ions Generally, apoenzymes of metalloenzymes can be used for the detection of the corresponding metal ion. Restoration of enzyme activity obtained in the presence of the metal ion can be correlated to its concentration. This principle has been demonstrated in the detection of copper while evaluating reconstituted catalytic activities in galactose oxidase and ascorbate oxidase and also in the detection of zinc since this ion is essential for the activity of carbonic anhydrase and alkaline phosphatase [416]. The need of stripping the metal for the preparation of the apoenz5une may demand tedious procedures and a catalytic assay with the addition of the substrate is always required for detection. 

Most of the known ribozymes require divalent cations, usually Mg +, which is used for proper assembly of a complex 3D structure that provides an appropriate environment for catalysis. Metal ions can also act as cofactors in the chemical reaction enabling catalysis by activation of the nucleophile, stabilization of the transition state and protonation of the leaving group in general acid catalysis [424]. Accordingly, ribozymes could be considered as metalloenzymes. This fact has suggested the use of ribozymes as sensing elements for the detection of metal ions. 

Two features of luminescence make it a powerful method for analytical detection of metal ions. First is sensitivity fluorimetric reagents have been developed for the detection of even nanomolar metal ion concentrations. Indeed, even single molecule fluorescence is possible. 

Figure 13.7 Indirect detection of metal ions. Stationary phase sulfonic acid derivatized polyst)n ene-divinylbenzene resin particles (5 im). Mobile phase 0.1 M Ce(lII) in water, 1 ml/inin. Sample 20 fiL containing (1) sodium, 1.6 ppm (2) potassium, 2.1 ppm (3) rubidium, 7.1 ppm (4) cesium, 12 ppm (5) magnesium, 2.5 ppm (6) calcium, 1.6 ppm. Reproduced with permission from J. H. Sherman and N. D. Danielson, Anal. Chem., 59 (1987) 490 (Fig. 3). 1987 American Chemical Society.

Fluorescent sensors have been widely investigated for the detection of many types of compounds and a particularly fruitful field is the selective detection of metal ions for example, a series (Zinpyr) of zinc sensors, based on fluorescein-containing attached chelating ligands, which can be used for quantitative determination of Zn or imaging of zinc-containing biological structures. ... 

P. Maitoza and D. C. Johnson, Detection of Metal Ions Without Interference from Dissolved Oxygen by Reverse Pulse Amperometry in Flow Injection Systems and Liquid Chromatography. Anal. Chim. Acta, 118 (1980) 233. 

---