Metal organic frameworks for chemical recognition

Abstract

The need for real-time, compact, and inexpensive chemical detectors has become more pressing in recent years. Homeland security and defense applications need effective portal monitoring, chemical weapons sensing, and water quality testing. Devices that can serve as personal exposure monitors, provide advance warning of food spoilage, and enable breath analyzers to uncover pre-symptomatic infection are also in demand. These applications pose many challenges because they require high levels of sensitivity and specificity in small, economical packages. Many existing technologies, such as mass spectrometry and chemiluminescence, lack the combination of sensitivity, selectivity, portability, and low cost needed for these applications. Micro electrical-mechanical systems (MEMS) offer a potential solution that can be cheaply mass produced. Microcantilever sensors have many of the desired characteristics and can be exquisitely sensitive platforms for chemical and biosensing.1 These devices require simple instrumentation, and arrays of cantilevers on a single chip can provide sensitivity to multiple analytes. Yet, such systems still need better recognition chemistries that can identify a broad range of analytes. The tailorable nanoporosity and ultrahigh surface area of metal organic frameworks (MOFs) make them ideal candidates for recognizing analytes in sensing applications. A typical MOF consists of metal cations, such as Zn(II), linked by anionic organic linker groups, such as carboxylates, yielding a rigid but open framework that accommodates guest molecules. An intriguing aspect of these frameworks is their adsorbateinduced structural flexibility.2 The unit cell dimensions of some MOFs can vary by as much as 10% when molecules are absorbed within their pores.3 This behavior suggests a signal transduction mechanism, in which an adsorbate induces Figure 1. The structure of theMOFHKUST-1 (green: copper; red: oxygen; black: carbon; white: hydrogen) enables stress-induced chemical detection when a thin layer is integrated on a microcantilever surface. In this view, the exchangeable axial coordination sites on the Cu(II) ions are unoccupied.