ABSTRACT:Here we have rationally designed a tunable DNA-based nanoswitch whose closing/opening can be triggered over specific different pH windows. This nanoswitch forms an intramolecular triplex DNA structure through pH-sensitive parallel Hoogsteen interactions. We demonstrate that by simply changing the relative content of TAT/CGC triplets in the switch we can rationally tune its pH-dependence over more than 5 pH units. By using a combination of such nanowitches with different pH sensitivity, we also demonstrate how we can engineer a pH nanosensor that can precisely monitor pH variations over 5.5 units of pH. With their fast response time (<200 msec) and high reversibility, these pH-triggered nanoswitches appear particularly suitable for applications ranging from the real-time monitoring of pH changes in-vivo to the development of pH sensitive smart nanomaterial.Nature often employs finely pH-regulated biomolecules to modulate and tune a number of biological activities 1 ranging from enzyme catalysis 2 to protein folding 3 , membrane function 4 and apoptosis 5 . For these reasons, developing probes, switches or nanomaterials that are able to respond to specific pH changes should prove of utility for several applications in the fields of in-vivo imaging, clinical diagnostics, and drug-delivery [6][7][8] .By taking advantage of the high versatility and designability of DNA chemistry 9-19 several groups have recently developed pH-triggered DNA-based probes or nanomachines [20][21][22][23][24][25][26][27][28][29][30] . Such probes typically exploit DNA secondary structures that display pH-dependence due to the presence of specific protonation sites. These structures include I-motif [21][22][23]26,29,31 , intermolecular triplex DNA 25,28,32 , DNA tweezers 20 and, more recently, the Amotif 33 . Despite the promising and advantageous characteristics of some of these DNA-based nanomachines, which include fast response times and sustained efficiency over several cycles, a drawback inevitably affects their performances: they all respond (with an exception 33a ) over a fixed pH window that typically spans 1.5 to 2 pH units 26,34,33b . These nanomachines, therefore, cannot be adapted to provide a useful output outside these fixed pH-windows.Here we describe a method to rationally design and program pH-triggered DNA-based nanoswitches whose pH-dependence can be finely tuned and modulated over more than 5 units of pH. We created our switches by taking advantage of the wellcharacterized pH sensitivity of the parallel Hoogsteen (T,C)-motif in triplex DNA [34][35][36] . To do so we have designed a DNAbased triplex pH-triggered nanoswitch that consists in a double intramolecular hairpin stabilized with both Watson-Crick (W-C) and parallel Hoogsteen interactions (Fig. 1). More specifically, one hairpin of the triplex nanoswitch is formed by the W-C hybridization of two 10-base complementary portions separated by a 5 base loop. This duplex DNA is then able to form a triplex structure via the formation of a second hairpin through...

We have demonstrated a novel sensing strategy employing single-stranded probe DNA, unmodified gold nanoparticles, and a positively charged, water-soluble conjugated polyelectrolyte to detect a broad range of targets including nucleic acid (DNA) sequences, proteins, small molecules, and inorganic ions. This nearly "universal" biosensor approach is based on the observation that, while the conjugated polyelectrolyte specifically inhibits the ability of single-stranded DNA to prevent the aggregation of gold-nanoparticles, no such inhibition is observed with double-stranded or otherwise "folded" DNA structures. Colorimetric assays employing this mechanism for the detection of hybridization are sensitive and convenient-picomolar concentrations of target DNA are readily detected with the naked eye, and the sensor works even when challenged with complex sample matrices such as blood serum. Likewise, by employing the binding-induced folding or association of aptamers we have generalized the approach to the specific and convenient detection of proteins, small molecules, and inorganic ions. Finally, this new biosensor approach is quite straightforward and can be completed in minutes without significant equipment or training overhead.biosensor | aptamer | visual detection | thrombin detection | cocaine detection G old nanoparticle colorimetric biosensors have seen significant applications in diagnostics, environmental monitoring, and antibioterrorism supporting unaided, visual readout (1-12). Commonly, the relevant nanoparticles are covalently modified with either a probe DNA or an aptamer such that hybridization (13-16) or aptamer-target interactions (17-27), for example the scanometric method developed by Mirkin (25), which is a very sensitive and specific tool, crosslink them, inducing aggregation. The second broad approach utilizes unmodified nanoparticles. (28-30) These two approaches, however, suffer from timeconsuming (20-40 h of assembly) and relatively poor (low nanomolar) detection limits, respectively. Here, a unique, colorimetric sensing strategy employing a simple but selective combination of a single-stranded DNA probe, a positively charged, water-soluble conjugated polyelectrolyte, and unmodified gold nanoparticles is demonstrated. The universality of this method allows detection of a broad range of targets, including nucleic acid (DNA) sequences, proteins, small molecules, and ino rganic ions. Our approach is rapid (turnaround time is 5-10 min) and sensitive (picomolar concentrations of target DNA are readily detected with the naked eye, even in complex sample matrices like blood serum). Hence, an operator with minimum scientific overhead can easily employ this technique.Generally, the gold nanoparticle applications typically rely on a quantitative coupling between target recognition and the aggregation of the nanoparticles, which, in turn, leads to a dramatic change in the photonic properties-and thus the color-of the nanoparticle solution. This colorimetric "readout" avoids the relative complexity inherent...

Recent years have seen the publication of both empirical and theoretical relationships predicting the rates with which proteins fold. Our ability to test and refine these relationships has been limited, however, by a Reprint requests to: Kevin W. Plaxco, Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA 93106, USA; e-mail: kwp@chem.ucsb.edu; fax: (805) 893-4120.Abbreviations: GuHCl, guanidine hydrochloride; tris, tris hydroxymethylaminoethane; HEPES, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; TCEP, tris(2-carboxyethyl)phosphine; CD, circular dichroism. Article published online ahead of print. Article and publication date are at

Binding-induced biomolecular switches are used throughout nature and, increasingly, throughout biotechnology for the detection of chemical moieties and the subsequent transduction of this detection into useful outputs. Here we show that the thermodynamics of these switches are quantitatively described by a simple 3-state population-shift model, in which the equilibrium between a nonbinding, nonsignaling state and the binding-competent, signaling state is shifted toward the latter upon target binding. Because of this, their performance is determined by the tradeoff inherent to their switching thermodynamics; while a switching equilibrium constant favoring the nonbinding, nonsignaling, conformation ensures a larger signal change (more molecules are poised to respond), it also reduces affinity (binding must overcome a more unfavorable conformational free energy). We then derive and employ the relationship between switching thermodynamics and switch signaling to rationally tune the dynamic range and detection limit of a representative structure-switching biosensor, a molecular beacon, over 4 orders of magnitude. These findings demonstrate that the performance of biomolecular switches can be rationally tuned via mutations that alter their switching thermodynamics and suggest a mechanism by which the performance of naturally occurring switches may have evolved.allostery ͉ ligand-induced conformational change ͉ pre-existing equilibrium ͉ rational design, sensitivity ͉ riboswitches

Biomolecular recognition has long been an important theme in artificial sensing technologies. A current limitation of protein- and nucleic acid-based recognition, however, is that the useful dynamic range of single-site binding typically spans an 81-fold change in target concentration, an effect that limits the utility of biosensors in applications calling for either great sensitivity (a steeper relationship between target concentration and output signal) or for the quantification of more wide-ranging concentrations. In response, we have adapted strategies employed by nature to modulate the input-output response of its biorecognition systems to rationally edit the useful dynamic range of an artificial biosensor. By engineering a structure-switching mechanism, we first generated a set of receptor variants displaying similar specificity, but spanning a wide range of target affinities. We then rationally combined sub-sets of these variants to expand the pseudo-log-linear dynamic range of our biosensor to six orders of magnitude. Using other combinations of variants we have also fabricated more elaborate, three-state dose-response sensors that respond sensitively only when the target concentration falls above or below some well-defined intermediate regime. Finally, by combining signaling and non-signaling receptor variants, we have succeeded in both compressing the dynamic range of our biosensor by an order of magnitude, and in rationally tuning its narrowed threshold response to any arbitrarily selected target concentrations. Given their widespread occurrence in nature, it would appear that these same approaches could significantly enhance the performance of many biomolecule-based technologies.