Nanomaterials for Biosensing Lipopolysaccharide

Abstract

Lipopolysaccharides (LPS) are endotoxins, hazardous and toxic inflammatory stimulators released from the outer membrane of Gram-negative bacteria, and are the major cause of septic shock giving rise to millions of fatal illnesses worldwide. There is an urgent need to identify and detect these molecules selectively and rapidly. Pathogen detection has been done by traditional as well as biosensor-based methods. Nanomaterial based biosensors can assist in achieving these goals and have tremendous potential. The biosensing techniques developed are low-cost, easy to operate, and give a fast response. Due to extremely small size, large surface area, and scope for surface modification, nanomaterials have been used to target various biomolecules, including LPS. The sensing mechanism can be quite complex and involves the transformation of chemical interactions into amplified physical signals. Many different sorts of nanomaterials such as metal nanomaterials, magnetic nanomaterials, quantum dots, and others have been used for biosensing of LPS and have shown attractive results. This review considers the recent developments in the application of nanomaterials in sensing of LPS with emphasis given mainly to electrochemical and optical sensing.

Keywords: biosensing; endotoxin; lipopolysaccharides (LPS); nanomaterials.

Figure 1

Lipopolysaccharide (LPS) transport pathway in Escherichia coli. LPS is synthesized on the cytoplasmic side of the inner membrane (IM) and flipped to the periplasmic side by the ATP-binding cassette (ABC) transporter MsbA. LPS is then transported to the cell surface by the LPS transport (Lpt) pathway. This pathway consists of seven essential proteins, LptA, LptB, LptC, LptD, LptE, LptF, and LptG. LPS is extracted from the IM in an ATP-dependent manner by the ABC transporter LptB2FG and transferred to LptC, which forms a complex with LptB2FG. LptC consists of a single membrane-spanning domain and a large periplasmic domain, which forms a periplasmic bridge with the soluble protein LptA and the amino-terminal region of LptD. LPS transverses the aqueous periplasmic space through this protein bridge and reaches the cell surface with the aid of the carboxy-terminal domain of LptD, which forms a β-barrel structure that is plugged by the outer membrane (OM) lipoprotein LptE. LPS is composed of lipid A, the inner and outer core oligosaccharides, and the O antigen, which is highly variable and absent in Escherichia coli K-12. The letters (A–G) in the figure correspond to the respective Lpt protein in the transport pathway. EtN, ethanolamine; Gal, d-galactose; Glc, d-glucose; Hep, l-glycero-d-manno-heptose; Kdo, 3-deoxy-d-manno-octulosonic acid; P, phosphate; Pi, inorganic phosphate. (Reproduced with permission from reference [2]).

Figure 2

(A) Representation of LPS structure. (B) Fluorescently labeled peptide coupled with graphene oxide acting as a biosensor for LPS by showing variation in intensity of fluorescence upon its interaction with LPS. (Reproduced with permission from reference [52]), copyright ACS).

Figure 3

Gold nanorods immobilized on functionalized glass substrates for sensing experiments for LPS. (Reproduced with permission from reference [64], copyright ACS).

Figure 4

Schematic representation of the PVM-AuNpCys-CramolL-BSA-LPS biosensor system. (Reproduced with permission from reference [65]). PVM: poly(vinyl chloride-co-vinyl acetate-co-maleic acid); BSA: bovine serum albumin.

Figure 5

Schematic diagram showing dye labeled DNA aptamer with magnetic nanoparticles (MNPs) and the change in signal on encountering target molecule (Reproduced with permission from reference [80]).

Figure 6

Sensor design and sequential steps to achieve surface biofunctionalization. (Reproduced with permission from reference [104]).