Localized Surface Plasmon Resonance (LSPR) Detection of Diphtheria Toxoid Using Gold Nanoparticle-Monoclonal Antibody Conjugates Mehdi Zeinoddini 1 & Azadeh Azizi 1 & Shiva Bayat 1 & Zeinab Tavasoli 2 Received: 26 July 2016 /Accepted: 15 February 2017 # Springer Science+Business Media New York 2017 Abstract Detection of diphtheria toxin (DT) which is pro- duced by Corynebacterium diphtheria, a zoonotic pathogen and a leading cause of diphtheria, is the critical step in the clinical laboratory. Traditional methods for DT detection are time consuming with low sensitivity. Herein, a localized sur- face plasmon resonance (LSPR) nanobioprobe has been de- veloped based on specific immunological interactions be- tween gold nanoparticles (GNPs) conjugated with monoclonal antibody and diphtheria toxoid in order for the rapid detection of DT. For this, plasmonic GNPs were conjugated to mono- clonal antibodies covalently. The covalent conjugation has been confirmed by dynamic light scattering (DLS) and elec- trochemical techniques. Then, structural alterations of the con- jugated antibody were monitored by circular dichroism (CD) and fluorescence spectroscopy methods. After that, the sensi- tivity of the nanobioprobe has been investigated via measur- ing the LSPR band λmax shifts of GNPs and LSPR sensitivity of nanobioprobe was compared with the ELISA method. Results suggested that this assay is highly selective and sen- sitive with a lower detection limit of about 10 ng/mL. The LSPR biosensor reduced the DT detection time from 2 or 3 days to less than 1 h compared with traditional methods. In conclusion, the investigation presents a rapid, sensitive, and selective method for the diagnosis of DT in clinical specimens. Keywords Localized surface plasmon resonance (LSPR) . Nanobioprobe . Anti-diphteria IgG . Circular dichroism spectroscopy . Electrochemistry Introduction The recent outreach of a wide variety of optical biosensors has been driven by progress in extremely sensitive optical trans- ducers along with the intensive specificity, affinity, and versa- tility of bio-molecular interactions [1, 2]. Clinical diagnosis, bio-molecular engineering, drug design, environmental con- trol, and the food industry are fields which apply these sensors [3]. Optical biosensors which are based on surface plasmon resonance (SPR) changes in thin gold films have widespread application [4]. Among various nanobiosensors, SPR-based nanobiosensors are considered as one of the most powerful tools in biotechnology and biosensor research [4, 5]. The priority of this technology is use of metallic nanostruc- tures with localized surface plasmon resonance properties in order to transfer information with simultaneous speed of light and electron via very fast and sensitive electrical sensor [5, 6]. SPR sensors which are commonly used to probe the kinetics and strength of binding interactions detect changes in the re- flectance intensity or angle of thin gold films and have been commercialized for nearly two decades [7]. Employing noble metal nanoparticles by localized surface plasmon resonance (LSPR) sensors are increasingly used as an alternative to SPR sensors because of the highly localized electromagnetic fields on nanoparticle surfaces which can enable improved detection of nanoscale biological analytes [5, 8]. Based on the LSPR of metal nanoparticles, plasmonic bio- sensors have been developed using both nanoparticle arrays and single nanoparticles [9]. * Azadeh Azizi azizi.a@modares.ac.ir 1 Department of Bioscience and Biotechnology, Malek-Ashtar University of Technology, Tehran, Iran 2 Department of Biophysics, Faculty of Biological Sciences, Tarbiat Modares University, Tehran, Iran Plasmonics DOI 10.1007/s11468-017-0548-7