Putrescine and cadaverine are toxic biogenic amines in spoiled food, which poses a serious threat to food security. In this work, we reported a highly sensitive three-dimensional (3D)-rosettelike surface-enhanced Raman spectroscopy (SERS) substrate functionalized with a p-mercaptobenzoic acid (p-MBA) monolayer to detect liquid and gaseous putrescine and cadaverine in pork samples. The SERS substrate was made by a combination of the merit of the 3D morphology of ZnO nanorod arrays on a flexible porous poly(vinylidene fluoride) (PVDF) membrane and the in situ chemical growth of Au nanoparticle seeds on Au film-coated ZnO nanorods, which produced a 3Drosettelike BigAuNP/Au/ZnO/P heterostructure with abundant SERS-active hot spots that significantly enhanced the localized surface plasmonic resonance (LSPR) effect and charge-transfer (CT) effect of Raman enhancement. This SERS substrate showed high sensitivity, reproducibility, stability, and uniformity. With the p-MBA molecular monolayer as the sensing interface, our SERS substrate realized the highly sensitive and quantitative detection of liquid putrescine and cadaverine within 10 min, with a limit of detection (LOD) of 3.2 × 10 −16 and 1.6 × 10 −13 M, respectively. Additionally, the sensor showed efficient SERS responses to gaseous amine molecules at low concentrations (putrescine: 1.26 × 10 −9 M, cadaverine: 2.5 × 10 −9 M). Further, the sensor was successfully applied to determine the total content of putrescine and cadaverine. Moreover, the practicability of this SERS sensor was verified by the measurement of liquid and gaseous amines in pork samples, and it showed great potential applications for sensitive detection of food spoilage.

We report what we believe to be the first Fourier domain mode-locked (FDML) opto-electronic oscillator (OEO) without using a tunable signal source to implement, such as a tunable laser or a tunable microwave source as described in the previous reports. We designed and fabricated a tunable microwave filter with individually packaged microwave components, in which a low cost diode-tuned phase shifter was used to rapidly tune the filter center frequency. We successfully realized Fourier domain mode-locking of an OEO using the diode-tuned filter and obtained linearly chirped microwave signals around 9 GHz with a chirp rate of 36 MHz/µs and a frequency tuning range of 0.4 GHz, which can be extended to 142 MHz/µs and 1.56 GHz, respectively, with a filter circuit using chip sized components. We found, for the first time to the best of authors’ knowledge, that the phase noise of FDML OEO’s delayed self-heterodyne signal is an excellent indicator for mode-locking frequency optimization, which had a “U” shape dependence on the detuning of the mode-locking frequency with a locking range of over 40 Hz. We also investigated harmonic mode-locking of the FDML up to 5th harmonics and achieved a chirp rate of 180 MHz/µs using the tunable filter of individually packaged components. Compared with the previous FDML OEO’s implemented with tunable signal sources, our diode tuned FDML OEO has the advantages of low cost, compact size, excellent frequency tuning linearity, easy implemention and immunity to laser frequency drift and noise for achieving better frequency repeatability and lower phase noise.

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