Metal–Organic Framework-Based Fiber Optic Sensor for Chromium(VI) Detection

Abstract

This study demonstrates a novel fiber optic sensing strategy for selective adsorption and rapid detection of Cr(VI) ions by exploiting a suitable metal−organic framework matrix and the characteristic spectral absorption of Cr(VI) at 395 nm wavelength, respectively. U-bent fiber optic sensor (U-FOS) probes that exhibit remarkably high evanescent wave-based absorbance sensitivity were employed to efficiently detect the Cr(VI) ions that are adsorbed to a stable zeolitic imidazolate framework (ZIF-67) matrix immobilized on the probe surface. A facile technique was developed for the fabrication of ZIF-67-coated U-FOS probes (FOS/ZIF-67) involving an in situ deposition process followed by heat treatment. Selectivity of the FOS/ZIF-67 probes to Cr(VI) was confirmed by optical absorption spectral investigations with 14 other heavy metals and interfering ions. The sensor performance was evaluated with a compact light-emitting diode-photodetectorbased setup. FOS/ZIF-67 probes demonstrate an ability to detect Cr(VI) ions with a limit of detection of 1 ppb and a wide linear dynamic range from 0.005 to 100 ppm within a short response time of 5 to 10 min. These sensors show good recovery rates with real water samples and a shelf-life of at least 4 weeks under ambient conditions, thereby demonstrating their viability for real-world application.

Cobalt-Doped ZIF-8 Nanoparticle-Decorated Fiber Optic Sensor for Copper Ion Detection

“…In addition to a rise in optical extinction at 400 nm, the EWA spectra show that an additional broad characteristic absorption band between 500 and 700 nm appeared for the [Cu(II)] > 20 ppm, although the naked-eye observations from the probes did not reveal any detectable change in the color of the MOF matrix over the probe (Figure b). Unlike our previous study on ZIF-67 FOS for Cr(VI) ion detection based on their characteristic absorption at 390 nm, these absorption bands do not match with the characteristic absorption of Cu(II) at 800 nm (Figure S6). This observation strongly points to the significant role of the other metal ions including Zn 2+ and Co 2+ present in the Co/ZIF-8 nanoparticle as a result of their interactions with Cu(II) ions.…”

“…The EWA spectra reveal a rapid deposition of Co/ZIF-8 within 4 min of incubation with a significant optical absorption as high as 1 AU (arbitrary units) at 400 nm (Figure S1), in comparison to its characteristic peak at 297 nm, as found in the solution-phase synthesis (Figure S2). The subsequent heat treatment of the Co/ZIF-8 FOS probes for 15 min gave rise to a stable MOF film over the fiber surface as confirmed by the exposure of the probes to DI water and methanol (Figure S3). To verify the scalability of the in situ deposition process, the Co/ZIF-8 FOS probes were fabricated offline (4 min in growth solution) and compared with the probes obtained by the online deposition process (Figure S4).…”

“…For example, we have recently demonstrated a portable zeolitic imidazolate framework-67 (ZIF-67) coated fiber optic absorption sensor for hexavalent chromium detection with high sensitivity and selectivity. 24 Some MOFs, including Zr−tetraphenylporphyrin tetrasulfonic acid hydrate−MOF (ZTM) 25 and zeolitic imidazolate framework-8 (ZIF-8) in conjunction with fluorescent nanomaterials such as CDs and AuNCs, 26,27 have been explored for selective detection of Cu(II) ion based on fluorescence quenching in solution-phase. In particular, pristine ZIF-8 and its doped variants have gained wide interest due to their facile synthesis routes and their stability.…”

“…40 In this study, we investigate Co/ZIF-8 as a novel sensitive, selective, and stable chemoreceptor and an alternate to pristine ZIF-8 for Cu(II) ion detection. We show the development of a dip-type sensor that works on the principle of attenuation of the light passing through the fiber by the Co/ZIF-8 nanoparticles on the fiber surface due to evanescent wavebased absorption upon exposure to Cu(II) ions 24,38,40 (Figure 1). A method for room temperature based rapid in situ deposition of Co/ZIF-8 over the FOS surface was developed by optimizing the molar ratio of precursors.…”

“…Indeed, such optical attenuation by the MOFs allows the realization of field-deployable heavy metal ion sensors with simpler optical instrumentation. For example, we have recently demonstrated a portable zeolitic imidazolate framework-67 (ZIF-67) coated fiber optic absorption sensor for hexavalent chromium detection with high sensitivity and selectivity …”