New Method for Accurate Determination of Polyphenol Oxidase Activity Based on Reduction in SERS Intensity of Catechol

Polyphenol oxidase inhibited by 4-hydroxycinnamic acid and naringenin: Multi-spectroscopic analyses and molecular docking simulation at different pH

It is still unclear how pH affects the inhibitory effects of phenolic acids and flavonoids on polyphenol oxidase (PPO). In this study, 4-hydroxycinnamic acid and naringenin were selected to investigate their interactions with PPO from pH 6.8 to 5.0. Results showed that acidic pH could enhance the inhibitory effect of inhibitors and a greater enhancement effect was observed in 4-hydroxycinnamic acid. Fluorescence emission spectra indicated that 4-hydroxycinnamic acid and naringenin interacted with PPO and quenched its intrinsic fluorescence, which was also enhanced by acidic pH. Circular dichroism suggested that 4-hydroxycinnamic acid and naringenin could reversibly bind to PPO molecules and transform α-helix into β-sheet. Molecular docking results revealed that 4-hydroxycinnamic acid and naringenin interacted with PPO through hydrogen bond and hydrophobic interaction, and more interactions were observed near the carboxyl group. These results indicated that acidic pH could significantly enhance the inhibitory effect of phenolic acid on PPO.

Surface-enhanced Raman scattering-based detection of plasmin activity by specific peptide substrate

In this study, a new surface-enhanced Raman scattering (SERS)-based method has been developed for the detection of plasmin activity. Firstly, different peptide sequences, which are specific to plasmin, were examined. Then, SERS substrates were prepared by chosen peptide substrate. Enzyme activity was determined by pursuing the reduction of DTNB band at 1331 cm^-1 with Raman spectroscopy. The reduction in SERS intensity was related to the plasmin activity, and changes in SERS intensity vs. plasmin concentration graph was obtained. Limit of detection (LOD) and limit of quantification (LOQ) values were calculated as 2.14 U/mL and 6.42 U/mL, respectively. Intra- and inter-day repeatability results were determined as 1.45% and 1.47% relative standard deviation (RSD). Also, recovery of the method was determined for the plasmin spiked milk samples. The results demonstrated that the proposed method could be successfully used to detect the plasmin activity in milk samples.

Adventitious root primordia formation and development in the stem of Ananas comosus var. bracteatus slip

There are about 4-6 slips on a fruit, and they are good materials for effective regeneration of Ananas comosus var. bracteatus. Adventitious root (AR) induction is essential for the propagation of Ananas comosus var. bracteatus slips. Growth regulator treatment, and culture medium are imperative factors that affect slip growth and rooting. In order to screen the optimal methods for slips rooting and reveal the anatomic procedure of slip rooting, this study induced slip rooting by different treatment of growth regulator, culture medium, observed the slip stem structure, AR origination and formation procedure through paraffin sections. The results showed that, slip cuttings treated with 100 mg/L of Aminobenzotriazole (ABT) for 6 hrs, cultured in river sand: coconut chaff: garden soil 2:2:1 medium is the optimal method for rooting. The proper supplementary of ABT can enhance the soluble sugar content, soluble protein content, polyphenol oxidase (PPO) activity and peroxidase (POD) enzyme activity, which resulted in the improvement of rooting. The slip stem structure is quite different from other monocots, which consists of epidermis, cortex, and stele with vascular tissues distributed in the cortex and stele. The AR primordia originates from the parenchyma cells located on the borderline between the cortex and stele. The vascular tissues in the AR develop and are connected with vascular tissue of the stem before the AR grew out the stem. The number of primary xylem poles in AR is about 30.

In situ and rapid determination of acetamiprid residue on cabbage leaf using surface-enhanced Raman scattering

BACKGROUND

Pesticide residues in agricultural products and foods pose a serious threat to human health, and therefore a simple, rapid and direct method is urgently needed for pesticide residue detection. In addition to realizing the detection of acetamiprid in cabbage extract solution, the main target of this study was to establish an in situ surface-enhanced Raman scattering (SERS) method, which could directly detect acetamiprid residue on cabbage leaf without the need for extraction. Acetamiprid was first used to contaminate the surface of fresh cabbage leaf, and then bimetallic silver-coated gold nanoparticles (Au@AgNPs) were added on the contaminated spots and dried for SERS measurement.

RESULTS

Results suggested that acetamiprid can be detected in cabbage extract and on cabbage leaf surface in situ using the SERS method based on the Au@AgNPs substrate. The limit of detection was 0.08 μg mL^-1 in cabbage extract and 0.14 mg kg^-1 on cabbage leaf, the recovery ranged from 80.5% to 105.5% and the relative standard deviation was in the range 4.37-10.63%.

CONCLUSIONS

The proposed SERS method provides an in situ, nondestructive and rapid way to detect acetamiprid residue on the surface of fruits and vegetables, which could serve as an auxiliary approach for early screening of contaminated produce in field or on site in the future. © 2020 Society of Chemical Industry.

Inhibitory mechanism of salicylic acid on polyphenol oxidase: A cooperation between acidification and binding effects

Salicylic acid is generally considered to combine with polyphenol oxidase (PPO) to inhibit activity and enzymatic browning, while its acidification effect on PPO activity was usually neglected. In this study, the inhibitory mechanism of salicylic acid on PPO was examined from acidification and binding effects by altering the buffer conditions. As the buffer concentration increased, contribution of acidification decreased while the binding effect became more predominant. Salicylic acid exhibited competitive inhibition on PPO, inducing the changes in secondary structure with a reduction in α-helix. Molecular docking results showed that salicylic acid interacted with residues HIS61, HIS85, HIS259, HIS263 and VAL283 through hydrogen bond and hydrophobic interaction. Furthermore, acidic pH enhanced the binding of salicylic acid to PPO with lower binding energy, additional hydrogen bond and electrostatic interactions. Therefore, both acidification and binding effects were important for salicylic acid on PPO inhibition and enzymatic browning control in fruit and vegetables.

An overview of surface-enhanced Raman scattering substrates by pulsed laser deposition technique: fundamentals and applications

Metallic nanoparticles (NPs), as an efficient substrate for surface-enhanced Raman scattering (SERS), attract much interests because of their various shapes and sizes. The appropriate size and morphology of metallic NPs are critical to serve as the substrate for achieving an efficient SERS. Pulsed laser deposition (PLD) is one of the feasible physical methods employed to synthesize metallic NPs with controllable sizes and surface characteristics. It has been recognized to be a successful tool for the deposition of SERS substrates due to its good controllability and high reproducibility in the manufacture of metallic NPs. This review provides an overview about the recent advances for the preparation of SERS substrates by PLD technique. The influences of parameters on the sizes and morphologies of metallic NPs during the deposition processes in PLD technique including laser output parameters, gas medium, liquid medium, substrate temperature, and properties of 3D substrate are presented. The applications of SERS substrates produced by PLD in the environmental monitoring and biomedical analysis are summarized. This knowledge could serve as a guideline for the researchers in exploring further applications of PLD technique in the production of SERS substrate.

Visualization of the in situ distribution of contents and hydrogen bonding states of cellular level water in apple tissues by confocal Raman microscopy

Raman spectroscopy has been employed for studying the hydrogen bonding states of water molecules for decades, however, Raman imaging data contain thousands of spectra, making it challenging to obtain information on water with different hydrogen bonds. In the current study, a novel method combining confocal Raman microscopy (CRM) imaging with the iterative curve fitting algorithms was developed to determine the distribution of water contents at the cellular level and water states with different hydrogen bonds in apple tissues. Raman imaging data ranging from 2700 to 3800 cm-1 were acquired from whole cells in the apple tissue, which were then decomposed into seven sub-peaks using the fixed-position Gaussian iterative curve fitting (FPGICF) algorithm. The content and hydrogen bonding states of cellular water were calculated as the area sum of the OH stretching vibration and the area ratio of DA-OH over DDAA-OH stretching vibration or the number of hydrogen bonds of each water molecule, respectively. Finally, the area of each sub-peak, the area sum of the OH stretching vibration, and the area ratio of DA-OH over DDAA-OH stretching vibration were used to visualize the distribution of each sub-peak, water contents and water states with different hydrogen bonds, respectively. In addition, it was found that the number of hydrogen bonds of each water molecule could also be considered as a criterion to describe the hydrogen bond states of water in apple tissues. The availability of such information should provide new insights for future study of cellular water in other food materials.

SERS detection of sodium thiocyanate and benzoic acid preservatives in liquid milk using cysteamine functionalized core-shelled nanoparticles

A cysteamine functionalized core shelled nanoparticles (Au@Ag-CysNPs) was presented for simultaneous and rapid detection of sodium thiocyanate (STC) and benzoic acid (BA) preservatives in liquid milk using surface-enhanced Raman spectroscopy (SERS) technique. A spectrum covering 350-2350 cm^-1 region was selected to detect STC with concentrations ranging from 0.5 to 10 mg/L and BA with concentrations ranging from 15 to 240 mg/L in milk samples. Characterization of nanoparticles using high-resolution TEM confirmed that the successful synthesis of Au@AgNPs with core (gold) size of 28 nm and shell (silver) thickness of about 5 nm was grafted with 120 μL of 0.1 nM cysteamine hydrochloride. Results showed that Au@Ag-CysNPs could be used to detect STC up to 0.03 mg/L with a limit of quantification (LOQ) of 0.039 mg/L and a coefficient of determination (R²) of 0.9833 in the milk sample. For detecting BA, it could be screened up to 9.8 mg/L with LOQ of 10.2 mg/L and R² of 0.9903. The proposed substrate was also highly sensitive and the employed method involved only minor sample pretreatment steps. It is thus hoped that the new substrate could be used in the screening of prohibited chemicals in complex food matrices in future studies.

A rapid dual-channel readout approach for sensing carbendazim with 4-aminobenzenethiol-functionalized core–shell Au@Ag nanoparticles

In this study, a 4-aminobenzenethiol-functionalized silver-coated gold nanoparticle (Au@Ag-4ABT NP) system was designed for the rapid sensing of carbendazim (CBZ) using a combination of naked-eye colorimetry and SERS dual-channel approach.

Development of Nanozymes for Food Quality and Safety Detection: Principles and Recent Applications

The public concerns about agrifood safety call for innovative and reformative analytical techniques to meet the inspection requirements of high sensitivity, specificity, and reproducibility. Enzyme-mimetic nanomaterials or nanozymes, which combine enzyme-like properties with nanoscale features, emerge as an excellent tool for quality and safety detection in the agrifood sector, due to not only their robust capacity in detection but also their attraction in future-oriented exploitations. However, in-depth understanding about the fundamental principles of nanozymes for food quality and safety detection remains limited, which makes their applications largely empirical. This review provides a comprehensive overview of the principles, designs, and applications of nanozyme-based detection technique in the agrifood industry. The discussion mainly involves three mimicking types, that is, peroxidase, oxidase, and catalase-like nanozymes, capable of detecting major agrifood analytes. The current principles and strategies are classified and then discussed in details through discriminating the roles of nanozymes in diverse detection platforms. Thereafter, recent applications of nanozymes in detecting various endogenous ingredients and exogenous contaminants in foods are reviewed, and the outlook of profound developments are explained. Evidenced by the increasing publications, nanozyme-based detection techniques are narrowing the gap to practical-oriented food analytical methods, while some challenges in optimization of nanozymes, diversification of recognition-to-signal manners, and sustainability of methodology need to conquer in the future.

Stable, Flexible, and High-Performance SERS Chip Enabled by a Ternary Film-Packaged Plasmonic Nanoparticle Array

The high sensitivity and long-term storage stability of a plasmonic substrate are vital for practical applications of the surface-enhanced Raman scattering (SERS) technique in real-world analysis. In this study, a rationally designed, ternary film-packaged, silver-coated gold-nanoparticle (Au@Ag NP) plasmonic array was fabricated and applied as a stable and high-performance SERS chip for highly sensitive sensing of thiabendazole (TBZ) residues in fruit juices. The ternary films played different roles in the plasmonic chip: a newborn poly(methyl methacrylate) (PMMA) film serving as a template for fixing the self-assembled closely packed monolayer Au@Ag NP array that provided an intensive hot spot, a fluorescent quantitative polymerase chain reaction adhesive film (qPCR film) acting as a carrier to retrieve the Au@Ag/PMMA film that was used to improve the robustness of the plasmonic array, and a polyethylene terephthalate (PET) film covered over the Au@Ag/PMMA/qPCR film performing as a barrier to improve the stability of the chip. The Au@Ag/PMMA/qPCR-PET film chip showed high sensitivity with an enhancement factor of 3.14 × 10⁶, long-term storage stability without changing SERS signals for more than 2 months at room temperatures, and a low limit of detection for sensing TBZ in pear juice (21 ppb), orange juice (43 ppb), and grape juice (69 ppb). In addition, the procedure for fabricating the Au@Ag/PMMA/qPCR-PET film SERS chip was easy to handle, offering a new strategy to develop flexible and wearable sensors for on-site monitoring of chemical contaminants with a portable Raman spectrometer in the future.

Recent advances in the detection of 17β-estradiol in food matrices: A review

Pollution of endocrine disrupting chemicals has become a global issue. As one of the hormonally active compounds, 17β-estradiol produces the strongest estrogenic effect when it enters the organism exogenously including food intakes, bringing potential harmfulness such as malfunction of the endocrine system. Therefore, in order to assure food safety and avoid potential risks of 17β-estradiol to humans, it is of great significance to develop rapid, sensitive and selective approaches for the detection of 17β-estradiol in food matrices. In this review, the harmfulness and main sources of 17β-estradiol are firstly introduced, followed by the description of the principles and applications of different approaches for 17β-estradiol detection including high performance liquid chromatography, electrochemistry, Raman spectroscopy, fluorescence and colorimetry. Particularly, applications in detecting 17β-estradiol in food matrices over the years of 2010-2018 are discussed. Finally, advantages and limitations of these detection methods are highlighted and perspectives on future developments in the detection methods for 17β-estradiol are also proposed. Although many detection approaches can achieve trace or ultratrace detection of 17β-estradiol, further studies should be focused on the development of in-situ and real-time methods to monitor and evaluate 17β-estradiol for food safety.

Ultrasensitive analysis of kanamycin residue in milk by SERS-based aptasensor

An ultrasensitive method for the kanamycin (KANA) detection in milk sample using surface-enhanced Raman spectroscopy-based aptasensor was employed in the current study. Double strand DNA binding bimetallic gold@silver nanoparticles were developed as a sensing platform. Probe DNAs were first embedded on the surface of gold nanoparticles by the end-modified thiol, and after silver shell encapsulating, KANA aptamer DNAs with the Raman reporter Cy3 were then hybridized with probe DNAs by complementary base pairing. Results showed that with increase in the KANA concentration, the Raman intensity of Cy3 decreased. Besides achieving selectivity, an ultralow detection limit of 0.90 pg/mL, a broad linear relationship ranging from 10 μg/mL to 100 ng/mL in aqueous reagent and satisfactory recoveries of 90.4-112% in liquid whole milk were obtained. The result of actual sample proved that this aptasensor was promising in trace determination of KANA residue.

Nanomaterial-based SERS sensing technology for biomedical application

Over the past few years, nanomaterial-based surface-enhanced Raman scattering (SERS) detection has emerged as a new exciting field in which theoretical and experimental studies of the structure and function of nanomaterials have become a focus.

Double strand DNA functionalized Au@Ag Nps for ultrasensitive detection of 17β-estradiol using surface-enhanced raman spectroscopy

A detection method for 17β-estradiol (E2) using surface-enhanced Raman scattering (SERS)-based aptamer sensor was presented. Raman reporter molecule Cy3 labeled E2-aptamer and DNA functionalized gold-silver core-shell nanoparticles (Au@Ag CS NPs) offered SERS with high sensitivity and selectivity. Based on the fabricated double strand DNA-immobilized gold-silver core-shell nanoparticles (Au@Ag NPs), SERS signal intensity of Raman reporter changed with the number of Cy3-labeled aptamer attached to the core-shell nanoparticles due to the strong binding affinity between the aptamers and E2 with different concentrations. A wide linear range from 1.0 × 10^-13 to 1.0 × 10^-9 was obtained for the detection of E2, with a low detection limit of 2.75 fM. This proposed method showed highly sensitive and selective for detecting E2, and could be used to determine E2 in actual samples.