Biosensors and Bioelectronics 31 (2012) 130–136 Contents lists available at SciVerse ScienceDirect Biosensors and Bioelectronics jou rn al h om epa ge: www.elsevier.com/locate/bios Multifunctional magnetic–plasmonic nanoparticles for fast concentration and sensitive detection of bacteria using SERS Lei Zhang a,1 , Jiajie Xu a,b,1 , Luo Mi a , Heng Gong b , Shaoyi Jiang a , Qiuming Yu a,∗ a Department of Chemical Engineering, University of Washington, Seattle, WA 98195, USA b The State Key Laboratory of Bioreactor Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China a r t i c l e i n f o Article history: Received 5 August 2011 Received in revised form 3 October 2011 Accepted 4 October 2011 Available online 12 October 2011 Keywords: Multifunctional nanoparticles Magnetic concentration SERS Bacteria detection a b s t r a c t Multifunctional magnetic–plasmonic Fe 3 O 4 –Au core–shell nanoparticles (Au-MNPs) were prepared for simultaneous fast concentration of bacterial cells by applying an external point magnetic field, and sen- sitive detection and identification of bacteria using surface-enhanced Raman spectroscopy (SERS). We demonstrated that a spread of a 10 L drop of a mixture of 10 5 cfu/mL bacteria and 3 g/mL Au-MNPs on a silicon surface can be effectively condensed into a highly compact dot within 5 min by applying an external point magnetic field, resulting in 60 times more concentrated bacteria in the dot area than on the spread area without concentration. Surrounded by dense uniformly packed Au-MNPs, bacteria can be sensitively and reproducibly detected directly using SERS. The principle component analysis (PCA) showed that three different Gram-negative bacterial strains can be clearly differentiated. We also demon- strated that the condensed multifunctional Au-MNPs dot can be used as a highly sensitive SERS-active substrate and a limit of detection better than 0.1 ppb was obtained in detection of small molecules such as 4-mercaptopyrine. This novel platform significantly simplifies the concentration and detection process, which holds great promise for applications in food safety, environmental monitoring, medical diagnoses, and chemical and biological threat detections. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Recently, growing interest has been paid to the develop- ment of multifunctional plasmonic magnetic nanoparticles by incorporation of gold nanostructures to superparamagnetic Fe 3 O 4 nanoparticles (MNPs) that can combine both plasmonic and mag- netic properties in one single nanoparticle (Melancon et al., 2009). Impinged by light, gold nanostructures such as nanodots, nanorods and nanoshells can generate extreme high local electric fields aris- ing from local surface plasmon resonance (LSPR). This is due to collective oscillation of conduction band electrons in response to the electric field of the electromagnetic radiation of the light. Both LSPR wavelength and electric field intensity can be tuned by vary- ing the dimensions of the nanostructures, the spacing between them and the surrounding dielectric media (Willets and Van Duyne, 2007). Therefore, metallic nanostructures have been used for bioimaging (El-Sayed et al., 2005; Jain et al., 2007; Tam et al., 2010), LSPR biosensing (Anker et al., 2008), surface-enhanced Raman spec- troscopy (SERS) (Yu et al., 2008), thermotherapy (Bardhan et al., 2009; Ji et al., 2007; Tam et al., 2010), and many other nanophotonic ∗ Corresponding author. Tel.: +1 206 543 4807; fax: +1 206 685 3451. E-mail address: qyu@u.washington.edu (Q. Yu). 1 These two authors contributed equally to this work. applications (Atwater and Polman, 2010; Schuller et al., 2010). Plas- monic MNPs have also been developed recently by many research groups with different methods. Several unique particle structures such as Fe 3 O 4 core/gold shell (Levin et al., 2009; Ma et al., 2009; Q. Zhang et al., 2010b), dumbbell-like NPs formed by attaching a Au NP to a Fe 3 O 4 NP (Yu et al., 2005), and gold nanorods deco- rated with Fe 3 O 4 NPs (Wang et al., 2009; Wang and Irudayaraj, 2010) have been developed. Tunable plasmonic properties were achieved by controlling the thickness of the gold nanoshell or the size of gold nanoparticles (Levin et al., 2009; Yu et al., 2005; Q. Zhang et al., 2010b). One of the fascinating aspects of plasmonic MNPs is that the plasmonic property can be further tuned by varying the inter-particle distance via an external magnetic field (Q. Zhang et al., 2010b). Because the sensitivity of SERS strongly depends on the distance of gold or silver nanoparticles, and single molecule detection can be achieved in the very narrow gap between nanopar- ticles (Kneipp et al., 1997; Nie and Emery, 1997), it is expected that closely packed plasmonic MNPs could serve as highly sensi- tive SERS-active substrates for sensing and detection applications. While plasmonic MNPs have been explored for protein concentra- tion/detection using SERS (Zhou et al., 2011), pathogen separation and imaging (Wang and Irudayaraj, 2010), photothermic therapy (Ma et al., 2009; Wang et al., 2009), drug delivery (Melancon et al., 2009; Xu et al., 2009), and immunoassay (Tang et al., 2006; Zhuo et al., 2009), little work has been reported to apply plasmonic MNPs 0956-5663/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2011.10.006