Flow-injection SPR has been demonstrated as a viable alternative for sensitive detection of heavy metal ions at trace levels [1-4]. The impetus behind using SPR for elemental analysis stems from its high sensitivity, simplicity, compact design (for possible field-based work), and universal detection mechanism (e.g., any elemental species adsorbed onto the SPR sensor can cause a detectable signal). The challenge, however, is to prepare a chemically or biologically modified sensor chip that is specific to a given analyte without interferences from other species present in the sample solution. Zare and coworkers have showed that an alkanedithiol-covered gold surface is selective to mercury ion (Hg2+) [1], and Forzani et al. attached oligopeptides onto gold films for selective determinations of Ni2+ and Cu2+ [2]. Zhang et al. recently constructed a sensor surface covered with apo-metallothionein (i.e., metal-free metallothionein, a cysteine-rich protein) for selective determinations of Hg2+ and Cd2+ [3]. The principle behind this detection mechanism is illustrated in FIG. 1. As metal ions are complexed by the apo-metallothionein (apo-MT) molecules, the resultant protein conformational changes can be sensitively measured by SPR. The extent of the conformational changes (or thickness variation of the immobilized proteins) is proportional to the amount of metal ions in the sample solution. This method yielded detection levels for Cd2+ and Hg2+ at 0.1 µM and 5 µM, respectively [3].

FIG. 1 Schematic representation of the conformation change of apo-MT upon metal binding. The black dots in (b) represent the metal ions. Notice that the predominant protein attachment is the linkage of MT molecules via their N-termini.