My research focuses on development and applications of the most advanced and innovative analytical techniques in solving the critical and emerging issues in food science. Surface-Enhanced Raman Spectroscopy (SERS) is a combination of Raman spectroscopy and nano-techniques. The use of metallic nanostructure enhances the Raman scattering more than a million times. We have developed various SERS based techniques for food safety, food chemistry, and food bioscience applications. Check out our recently published SERS review paper:
Jinkai Zheng and Lili He*. 2014. Surface-Enhanced Raman Spectroscopy for the Chemical Analysis of Food. Comprehensive Reviews in Food Science and Food Safety. 13 (3), 317-328. http://onlinelibrary.wiley.com/doi/10.1111/1541-4337.12062/abstract
1. Food safety applications: Development of rapid detection methods for targets of chemical, biological, and engineered nanoparticle (ENPs) contaminants in complex food matrices
Food contamination problems have become globalized and are the causes of many health problems and economic losses. There is an increasing need for rapid detection of food contaminants (chemicals and microbes) in foods, as traditional detection methods such as plating for microbes and HPLC-MS for chemicals are usually time-consuming. Recently, ENPs are considered as emerging contaminants to the environment and food system. However, there is lack of an analytical technique that can accurately and reliably detecting and quantifying ENPs, especially unstable ones like silver nanoparticles (AgNPs) in complex agriculture and food system. Therefore rapid and reliable detection methods are critically needed for the detection of these contaminants. The objective of my research is to develop rapid and reliable SERS based methods for rapid detection of chemical, biological, and ENPs in complex food matrices.
1.1 Innovative SERS methods for rapid detection of pesticides on and in food
We developed several innovative SERS methods for rapid detection of pesticides on and in different food matrices. For example, we developed a swab method that can recover pesticides from the food surfaces and then released them in organic solvents and mixed with SERS substrates for measurement. Alternatively, we can directly drop the gold nanoparticles (AuNPs) on the surfaces of plants for in situ detection (figure 1). For liquid food matrices, such as apple juice, an innovative aptamer-based method was developed to capture the specific pesticides out of the matrices before detection (figure 2). A portable Raman device was evaluated for onsite measurement of pesticides using SERS. Recently, we developed an in situ method to analyze pesticide penetration in plant tissues using AuNPs as probes (figure 3).
Figure 1. In situ detection of pesticides on tea leaves using SERS
Figure 2. Aptamer-based SERS method for detection of pesticides in apple juice
Figure 3. In situ analysis of pesticide penetration in plant leaves
1.2 SERS mapping for detection, identification, and quantification of multi-bacterial cells
Bacteria are our recent targets for SERS detection. A mapping method was developed to detect, identify and quantify multi-bacterial cells on Ag dendrites (figure 4). We also studied antibiotic-bacterial interactions using SERS.
Figure 4. Mapping multi-bacterial cells on the Ag dendrites
1.3 innovative SERS approaches to detect metallic ENPs
While most SERS detection studies are based on the unitization of Au and/or Ag nanostructures to enhance the signal of target analytes, I am the first to explore innovative application of SERS to the detection of AuNPs and AgNPs rather than the analytes. The detection is based on either the intrinsic signals from the surrounding chemicals or the reporter molecules. So far, we developed two innovative SERS methods to detect noble metal NPs in liquid and semi-liquid matrices (figure 5), and on and in plant leaves (figure 6).
Figure 5. Ferbam-based SERS to detect AgNPs and discriminate between other Ag species.
Figure 6. SERS mapping of AuNPs on and in leaves.
2. Food Chemistry applications: Characterization of food components and their interactions
Collaborating with other faculties and food industry, we have developed various novel SERS approaches to study food components and their interactions. For example, we have fabricated lipophilic gold nanoparticles which are compatible with lipids. Those lipophilic gold nanoparticles can enhance lipid signals in situ, and have been demonstrated to be more effective for studying lipid oxidation compared with conventional methods. We also fabricated amphiphilic gold nanoparticles and applied them in characterization of interfacial properties of emulsions (figure 6). The application of this innovative method will provide important structural information that cannot be obtained using existing analytical methodologies. Besides, we have characterized various polymethoxyflavons and their interactions with food proteins. We also collaborated with PepsiCo Inc. to study the solubility enhancement mechanism of two sweeteners prepared using a solid dispersion technique (figure 7).
Figure 7. Fabrication of amphiphilic AuNPs for characterizing emulsion interface
Figure 8. Possible interactions between Reb D and potassium sorbate at different ratios
USDA-NIFA, DHS-NCFPD, NIH-SBIR with Aglitron Inc., PepsiCo Inc., , American River Nutrition Inc.