Abstract
Magnetic nanoshells with tailored surface chemistry can enhance bacterial detection and separation technologies. This work demonstrated a simple technique to detect, capture, and aggregate bacteria with the aid of end-functionalized polyclonal antibody gold-coated magnetic nanoshells (pAb-Lis-AuMNs) as surface-enhanced Raman spectroscopy (SERS) probes. Listeria monocytogenes were used as the pathogenic bacteria and the pAb-Lis-AuMNs, 300 nm diameter, were used as probes allowing facile magnetic separation and aggregation. An optimized covalent bioconjugation procedure between the magnetic nanoshells and the polyclonal antibody was performed at pH six via a carbodiimide crosslinking reaction. Spectroscopic and morphological characterization techniques confirmed the fabrication of stable pAb-Lis-AuMNs. The resulting pAb-Lis-AuMNs acted as a SERS probe for L. monocytogenes based on the targeted capture via surface binding interactions and magnetically induced aggregation. Label-free SERS measurements were recorded for the minimum detectable amount of L. monocytogenes based on the SERS intensity at the 1388 cm−1 Raman shift. L. monocytogenes concentrations exhibited detection limits in the range of 104–107 CFU ml−1, before and after aggregation. By fitting these concentrations, the limit of detection of this method was ∼103 CFU ml−1. Using a low-intensity magnetic field of 35 G, pAb-Lis-AuMNs aggregated L. monocytogenes as demonstrated with microscopy techniques, including SEM and optical microscopy. Overall, this work presents a label-free SERS probe method comprised of a surface-modified polyclonal antibody sub-micron magnetic nanoshell structures with high sensitivity and magnetic induced separation that could lead to the fabrication of multiple single-step sensors.
Highlights
L. monocytogenes is a well-known foodborne pathogen with a ubiquitous presence in the environment, complicating its elimination from food production processes (Thakur et al, 1991; Law et al, 2015; Buchanan et al, 2017; Liu et al, 2017)
This study describes the functionalization of gold-coated iron oxide nanoshells (AuMNs) with a polyclonal L. monocytogenes antibody for use as a SERS-based biosensor through a covalent coupling surface functionalization step
After surface functionalization of the gold coated iron oxide nanoparticles (AuMNs) with polyclonal antibody (pAb)-Lis, the nanoshell diameter increased to 302 ± 7 nm as observed with Transmission Electron Microscopy (TEM)
Summary
L. monocytogenes is a well-known foodborne pathogen with a ubiquitous presence in the environment, complicating its elimination from food production processes (Thakur et al, 1991; Law et al, 2015; Buchanan et al, 2017; Liu et al, 2017). Consumption of L. monocytogenes-contaminated foods can cause listeriosis, exhibiting a high fatality rate ranging between 20 and 30% in high-risk individuals (Hoffmann et al, 2012). Food products with L. monocytogenes count levels higher than 100 colony-forming units (CFU)/g have been unacceptable for human consumption in Europe (Food and Authority, 2013). In the United States, any detection of L. monocytogenes CFU per 25 g of ready-to-eat food products is unacceptable, a practice referred to as the zero-tolerance policy (U.S FDA, 2020). Current high specificity detection methods with sensing limits acceptable for this range can take longer than 1 day for results (Mandal et al, 2011). The development and optimization of rapid and sensitive detection methods for L. monocytogenes can be extremely beneficial to prevent foodborne outbreaks
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