Effect of alkanethiol molecular structure on sensitivity of surface plasmon resonance sensor

Abstract This work aims to find the key factor determining the detection limit for an indirect competitive inhibition immunoassay using surface plasmon resonance (SPR) sensing. In our previous work, the thiol solution concentration used in a self-assembly process highly affected the alkanethiol monolayer structure on the sensor surface (Suherman et al., 2014). It was noticed that the monolayer structure determined the immunoassay sensitivity due to the orientation and the surface concentration of antigen in domain structure of monolayer. To study the effect of orientation of antigen, we examined here three types of alkanethiol compounds: dithiobis(succinimidyl undecanoate) (DSU; straight-chain alkanethiol), carboxy-EG 6 -undecanethiol (CEG 6 ; flexible-chain alkanethiol), and 3,3′-dithiobis[ N -(5-amino-5-carboxypentyl)propionamide- N , N′ -diacetic acid] (C 2 -NTA; three-branched alkanethiol). By electrochemical reductive desorption, the surface concentration of DSU, CEG 6 , and C 2 -NTA were estimated to be 6.6 × 10 −10 , 8.5 × 10 −10 , 4.6 × 10 −10  mol cm −2 , respectively. Subsequently, SPR suggested that the ratios of immobilized antigen (clenbuterol) per thiol were 0.13, 0.16, and 0.15 for DSU, CEG 6 , and C 2 -NTA, respectively. This suggests that the amount of immobilized clenbuterol was defined by the molecular size of clenbuterol. Sensor surface structures on a molecular scale were evaluated using high-resolution scanning tunneling microscopy (STM), which showed tilted clenbuterol with θ DSU  >  θ CEG6  >  θ C2-NTA . For SPR sensing of clenbuterol, C 2 -NTA showed the highest sensitivity (LOD = 10 ppt) among the examined alkanethiols, because clenbuterol formed a laid-on structure. Based on kinetics, the key factor for sensing performance is discussed.