Nano Today (2012) 7, 6—9
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanotoday
NEWS AND OPINIONS
Self-assembled nanorod supercrystals for ultrasensitive SERS diagnostics Ramón A. Alvarez-Puebla a,∗, Eugene R. Zubarev b,∗, Nicholas A. Kotov c,∗, Luis M. Liz-Marzán a,∗ a
Departamento de Química-Física, Universidade de Vigo, 36310 Vigo, Spain Department of Chemistry, Rice University, Houston, TX 77005, USA c Departments of Chemical Engineering, Materials Science and Engineering, Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109, USA b
Received 17 October 2011; received in revised form 20 October 2011; accepted 15 November 2011 Available online 15 December 2011
KEYWORDS SERS; Nanorods; Supercrystals; Diagnosis; Prions
Summary The fabrication of highly optically active supercrystals of anisotropic nanorods exploiting the electric field concentration and the nanoantenna effects provides a new family of optical sensors with the potential to maximize the SERS signal and thereby the possibility of detecting and quantifying the disease markers with low SERS cross-sections at ultralow concentrations. The capabilities of the new self-assembled nanorod SERS substrates have been demonstrated for real-time sensing of prions in real blood. It may also be possible to functionalize the top layers of supercrystals with specific recognition molecules for sensing many other disease markers, or even its integration into on-line devices, for the ultrasensitive screening of analytical targets relevant to medical science, environment, and homeland security. © 2011 Elsevier Ltd. All rights reserved.
One of the biggest challenges for nanoscience and nanotechnology is their application to the biomedical sciences. In the particular case of nanophotonics, several applications have been developed including plasmonic rulers [1], controlled drug release [2], or photothermal therapy [3], among
∗
Corresponding authors. E-mail addresses:
[email protected] (R.A. Alvarez-Puebla),
[email protected] (E.R. Zubarev),
[email protected] (N.A. Kotov),
[email protected] (L.M. Liz-Marzán).
others. However, some of the applications with the greatest potential impact are related to biosensing based on surface enhanced spectroscopies, especially SERS. SERS is an ultrasensitive technique with the ability to reach levels of sensitivity close to a single molecule, while providing all the structural information of the target molecule at biological conditions of pressure, temperature, and chemical environment. These exceptional sensing capabilities have been applied, directly or indirectly, to the structural characterization, dynamics, and ultrasensitive detection of several biologically
1748-0132/$ — see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.nantod.2011.11.001
Self-assembled nanorod supercrystals for ultrasensitive SERS diagnostics
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Figure 1 (A) SEM image of a typical nanorod supercrystal. (B) Electric field enhancement maps at the top layer of single and a three layer rod-stacked supercrystal, respectively. (C) Prion ultradetection in human blood. SERS spectra of (a) natural and (b) spiked human blood; (c) natural and (d) spiked human plasma. (e) SERS spectrum of the spiked plasma after spectral subtraction of the matrix (human plasma). (f) SERS spectrum of the prion.
relevant small metabolites indicative of disease or drug abuse such as hormones, sugars, nucleic acids, and peptides/proteins. Still, this spectroscopic technique can currently only be applied to certain specific problems while a massive number of metabolites, genes, and proteins have to be analytically targeted. The main reason for this is that the target analyte is usually present in a highly diluted state in a very complex solution (i.e. blood, plasma, serum, urine, saliva, cerebrospinal fluid, etc.) and the rest of the components of the fluid usually interfere with the signal acquisition. Further, several molecular targets, and especially those of polymeric nature, have a low SERS cross-section, hindering further their recognition in such complicated biological environments. The compositional complexity of the sample can be overcome by using specific molecules with the ability to selectively sequestrate a given metabolite, nucleic acid or peptide/protein, thus functioning as a biological interface between the targeted chemical moiety and the plasmonic surface, which provides the electric field required for signal enhancement. Examples of this can be found in the use of DNA/RNA fragments [4] that selectively recognize their counterparts, antibodies [5] or, more recently, aptamers [6]. These interfaces are usually rather thick, from several to tens of nanometers. Bearing in mind that SERS is a first layer effect showing an exponential decrease of the signal enhancement with the distance between the analyte and the surface, the detected signal is the change
in the vibrational fingerprint of the interface upon reaction with the target analyte. This usually results in a loss of sensitivity as either DNA/RNA, peptide/proteins or aptamers have a moderate SERS cross-section. However, this is not the only drawback of these approaches. First, the use of selective interfaces requires their availability, which in the case of antibodies and aptamers is not only difficult, but also expensive. Second, there are several important pathologies where the disease marker is essentially a different conformer of an endogenous biomolecule, thus preventing the use of immunological methods for their detection. One example of this is the family of diseases caused by prions, the smallest known infectious agents that cause a number of fatal neurodegenerative diseases in mammals such as bovine spongiform encephalopathy (BSE), scrapie, and Creutzfeldt—Jakob disease (CJD), which can only be diagnosed by postmortem brain biopsy [7]. These pathogen entities, the prions, have a specific amino acid sequence, -Met-Lys-His-Met-, that presents an extraordinary tendency to chelate gold [8], which can and has been exploited for presymptomatic detection in blood [9]. Unfortunately, SERS detection of these species is difficult as prions are small proteins and their cross-section is very modest, while the detection limits required for an effective diagnostic system are in the attomolar concentration range. Thus, in order to create a reliable SERS detection method, new concepts in plasmonic surface fabrication are needed. In this scenario, the recent
8 discovery of nanoantenna effects [10], i.e. the electric field concentration on the apexes of anisotropic plasmonic particles [11], the electric field transmission in ordered nanostructures [12] and, finally, the ability to prepare highly ordered nanoparticle superlattices (also known as supercrystals, Fig. 1A) using a simple selfassembly approach [13], has made possible the engineering of a novel optical platform with unique characteristics regarding both the intensity of the obtained signal and the homogeneity at a given analyzed spot on such a surface. While fabrication of supercrystals with nanospheres has been reported, quasi-monodisperse gold nanorods were preferred for the production of a highly efficient SERS platform. This preference is based on the above mentioned nanoantenna effect and the recently demonstrated possibility of accumulation of the electric field at the ends of an ordered structure. In fact, finite difference time domain (FDTD) theoretical calculations strongly support this idea, as illustrated in Fig. 1B, where the simulations for single layer and three-layer supercrystals show that the electric field at the top layer is nearly one order of magnitude higher for the three-layer crystal. Another interesting property of these photonic assemblies relies on their remarkable geometrical stability originating in thermodynamically favorable self-assembly pattern of gold nanorods, which also manifests in simplicity of their production. It is well known that appropriate surface functionalization is essential for the fabrication of supercrystals. One of the most common surfactants for this task is the surfactant cetyltrimethylammonium bromide (CTAB). However, it is also known that CTAB readily hinders the adsorption of analytes onto the plasmonic surfaces. This problem can be resolved by plasma etching of the organic material on gold surfaces, resulting not only in an outstanding increase in the SERS signal, but also in the regeneration of previously used substrates for new experiments with no loss of efficiency. We believe that this is a promising approach because we have demonstrated its abilities to detect small amounts of prions in blood, in equivalent concentrations to those of presymptomatic infection. Fig. 1C shows the SERS spectra of blood and plasma before and after addition of prions. Direct analysis of blood does not offer any insights about the presence of the infection. Nevertheless, after centrifugation of the sample to obtain plasma, both scrambled and correctly folded variants of peptides representing prions can be clearly identified in concentrations as low as attomolar. In our opinion, this new family of optical sensors, combining high electric field concentration and nanoantenna effects, have the potential to maximize the SERS signal and thereby detecting and quantifying disease markers with low SERS cross-sections at ultralow concentrations. Although this system has been demonstrated for direct sensing of prions in real blood, it may also be possible to functionalize the top layers of supercrystals with specific recognition molecules for indirect sensing, or even its integration into on-line devices, for the ultrasensitive screening of analytical targets relevant to medical science, environment, and homeland security.
R.A. Alvarez-Puebla et al.
Acknowledgements R.A.A.-P. acknowledges the RyC Program (MEC, Spain). This work was funded by the Spanish Ministerio de Ciencia e Innovacion (MAT2007-62696 and MAT2008-05755) and the Xunta de Galicia (PGIDIT06TMT31402PR and 08TMT008314PR). N.A.K. thanks research grants from NSF (R8112-G1, ECS0601345, 0932823), NIH (1R21CA121841 and 5R01EB007350) and DARPA W31P4Q-08-C-0426. Support for E.R.Z. was provided by the Robert A. Welch Foundation (C-1703), NSF (DMR-0547399), and Alliance for Nanohealth (W8XWH-07-20101).
References [1] N. Liu, M. Hentschel, T. Weiss, A.P. Alivisatos, H. Giessen, Science 332 (2011) 1407. [2] M. Yang, R.A. Alvarez-Puebla, H.S. Kim, P. Aldeanueva-Potel, L.M. Liz-Marzán, N.A. Kotov, Nano Lett. 10 (2010) 4013. [3] A.M. Gobin, M.H. Lee, N.J. Halas, W.D. James, R.A. Drezek, J.L. West, Nano Lett. 7 (2007) 1929. [4] A. Barhoumi, D. Zhang, F. Tam, N.J. Halas, J. Am. Chem. Soc. 130 (2008) 5523. [5] M. Sanles-Sobrido, et al., Nanoscale 1 (2009) 153. [6] N.H. Kim, S.J. Lee, M. Moskovits, Nano Lett. 10 (2010) 4181. [7] S.B. Prusiner, Proc. Natl. Acad. Sci. U.S.A. 95 (1998) 13363. [8] S. Lehmann, Curr. Opin. Chem. Biol. 6 (2002) 187. [9] R.A. Alvarez-Puebla, et al., Proc. Natl. Acad. Sci. U.S.A. 108 (2011) 8157. [10] G.W. Bryant, F.J. García de Abajo, J. Aizpurua, Nano Lett. 8 (2008) 631. [11] A.R. Alvarez-Puebla, L.M. Liz-Marzán, F.J. García de Abajo, J. Phys. Chem. Lett. 1 (2010) 2428. [12] A.I. Denisyuk, et al., Nano Lett. 10 (2010) 3250. [13] D.V. Talapin, J.S. Lee, M.V. Kovalenko, E.V. Shevchenko, Chem. Rev. 110 (2010) 389. Ramón A. Alvarez-Puebla is currently a Research Scientist at the Department of Physical Chemistry, University of Vigo. He worked as a postdoc in the Department of Chemistry and Biochemistry of the University of Windsor (Canada) with Prof. Ricardo Aroca and he was appointed as Research Officer at the National Institute for Nanotechnology of the National Research Council of Canada. He has co-authored over 80 articles and holds 2 patents. His current interests involve electronic and vibrational spectroscopy, surface enhanced spectroscopy and their application for sensor fabrication. Eugene R. Zubarev received his B.S. degree in Chemistry from Moscow State University in 1993 and Ph.D. in Chemistry from Russian Academy of Sciences in 1997. He then worked as a postdoctoral fellow at the University of Illinois in the group of Samuel I. Stupp before moving to Iowa State University in 2002 where he started his independent career as an Assistant Professor of Materials Science and Engineering. In 2005 he moved to Rice University Chemistry Department and was promoted to Associate Professor with tenure in 2009. He received the NSF Career Award in 2006 and Alfred P. Sloan Research Fellowship in 2008. His current research is focused on molecular
Self-assembled nanorod supercrystals for ultrasensitive SERS diagnostics
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self-assembly, synthesis of one-dimensional metallic nanocrystals, and hybrid nanomaterials.
provided expertise in self-assembly and nanoscale engineering of the optical properties of nanostructures.
Nicholas A. Kotov’s area of expertise is the synthesis and self-assembly of nanoparticles from different materials. He is the pioneer of self-organization processes in anisotropic metal, semiconductor, and oxide nanoparticles. His works included development of experimental approaches to observation of such processes, theoretical background, and applications of semiconductor self-assembling devices. Based on the layer-by-layer assembly approach of control of nanoscale organization of materials Prof. Kotov also developed a new family of composites with tunable mechanical, optical, and electrical properties and record characteristics. They are currently utilized in several biomedical devices and personal protection for military personnel. Prof. Kotov is a recipient of multiple international, national, state, and university awards. For this project he
Luis M. Liz-Marzán is a Ph.D. from the University of Santiago de Compostela (1992) and has been postdoc at Utrecht University and (more recently) visiting professor at Tohoku University, Michigan, Melbourne and Hamburg. He holds a chair in Physical Chemistry at the University of Vigo, where he is head of the Colloid Chemistry Group. He is co-author of over 230 publications and 5 patents, has received several research awards, and is editor and editorial advisory board member of several chemistry, nanotechnology and materials science journals. His current interests include nanoparticle synthesis and assembly, nanoplasmonics, and development of nanoparticle-based sensing and diagnostic tools.
