Biosensors and Bioelectronics 21 (2006) 1859–1866 Labeling tumor cells with fluorescent nanocrystal–aptamer bioconjugates Ted C. Chu a,1 , Felice Shieh b,1 , Laura A. Lavery a , Matthew Levy a , Rebecca Richards-Kortum c , Brian A. Korgel b,∗ , Andrew D. Ellington a,∗ a Institute for Cellular and Molecular Biology, University of Texas at Austin, Austin, TX 78712, United States b Department of Chemical Engineering, Center for Nano- and Molecular Science and Technology, Texas Materials Institute, University of Texas at Austin, Austin, TX 78712, United States c Department of Bioengineering, Rice University, Houston, TX 77251-1892, United States Received 19 September 2005; received in revised form 6 December 2005; accepted 6 December 2005 Available online 21 February 2006 Abstract Aptamers that bind to prostate specific membrane antigen (PSMA) were conjugated to luminescent CdSe and CdTe nanocrystals for cell-labeling studies. The aptamer–nanocrystal conjugates showed specific targeting of both fixed and live cells that overexpressed PSMA. More importantly, aptamers were able to label cells dispersed in a collagen gel matrix simulating tissue. The specific binding abilities and synthetic accessibility of aptamers combined with the photostability and small size of semiconductor nanocrystals offers a powerful and general tool for cellular imaging. © 2006 Published by Elsevier B.V. Keywords: Nanocrystal; Aptamer; Cell imaging 1. Introduction Prostate cancer affects approximately 1 in 11 men and it is the second leading cause of cancer deaths among American males (Greenlee et al., 2001). As with all cancers, early detection offers the best prospects for patient survival. Prostate cancer screening currently relies on rectal examinations to detect anomalies in the prostate gland, along with blood tests for upregulated prostate specific antigen (PSA) levels (Nelson, 2002). However, rectal examinations are invasive, and blood tests can sometimes be inconclusive, as PSA levels in the blood may be heightened by factors other than carcinoma (Untergasser et al., 2005). Taken as a whole, these diagnostic methods are prone to significant false- positive rates and exhibit relatively poor sensitivity (Postma and Schroder, 2005). In contrast to PSA, prostate specific membrane antigen (PSMA), a membrane-bound glycoprotein, is overex- pressed in many prostate cancers (Nelson, 2002). More specif- ically, high levels of PSMA have been found in patients with hormone-insensitive prostate cancer cells (Bostwick et al., 1998; ∗ Corresponding authors. Tel.: +1 512 471 5633; fax: +1 512 471 7060. E-mail addresses: korgel@mail.che.utexas.edu (B.A. Korgel), andy.ellington@mail.utexas.edu (A.D. Ellington). 1 These authors contributed equally to this work. Kawakami and Nakayama, 1997) as well as in the neovascula- ture associated with other solid malignant tumors (Liu et al., 1997). This well-characterized, integral membrane protein has therefore been identified as a good indicator of cancer growth and metastases. In fact, a variety of anti-PSMA-based therapies are currently under investigation (Chang, 2004; Slovin, 2005; Tricoli et al., 2004). One approach to PSMA detection has been to target fluores- cent markers to tumors via monoclonal antibodies (Barren et al., 1998; Liu et al., 2002). While nanocrystal–antibody conju- gates against PSMA have previously been used to image cancer cells implanted in live mice (Gao et al., 2004), the large size and immunogenicity of antibodies may limit their pharmaco- logical value. Humanized antibodies, antibody fragments, and short peptides isolated from processes such as phage display are also possible affinity reagents, but are similarly susceptible to peptidase fragmentation and immune response (Jayasena, 1999). Selected nucleic acid binding species (aptamers) are alter- native affinity reagents (reviewed in Jayasena, 1999). Aptamers have previously been selected against a variety of targets ranging from small molecules to proteins to cell surfaces. Aptamers typ- ically bind their targets with nanomolar or better affinities and have specificities comparable to those of monoclonal antibod- ies (Jenison et al., 1994). Moreover, modified nucleotides can be introduced into aptamers that render them highly resistant to 0956-5663/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.bios.2005.12.015