Amplified and Multiplexed Detection of DNA Using the Dendritic

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Amplified and Multiplexed Detection of DNA Using the Dendritic Rolling Circle Amplified Synthesis of DNAzyme Reporter Units Fuan Wang,‡ Chun-Hua Lu,‡ Xiaoqing Liu, Lina Freage, and Itamar Willner* Institute of Chemistry, The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel S Supporting Information *

ABSTRACT: The amplified, highly sensitive detection of DNA using the dendritic rolling circle amplification (RCA) is introduced. The analytical platform includes a circular DNA and a structurally tailored hairpin structure. The circular nucleic acid template includes a recognition sequence for the analyte DNA (the Tay-Sachs mutant gene), a complementary sequence to the Mg2+-dependent DNAzyme, and a sequence identical to the loop region of the coadded hairpin structure. The functional hairpin in the system consists of the analyte-sequence that is caged in the stem region and a single-stranded loop domain that communicates with the RCA product. The analyte activates the RCA process, leading to DNA chains consisting of the Mg2+-dependent DNAzyme and sequences that are complementary to the loop of the functional hairpin structure. Opening of the coadded hairpin releases the caged analyte sequence, resulting in the dendritic RCA-induced synthesis of the Mg2+-dependent DNAzyme units. The DNAzyme-catalyzed cleavage of a fluorophore/ quencher-modified substrate leads to a fluorescence readout signal. The method enabled the analysis of the target DNA with a detection limit corresponding to 1 aM. By the design of two different circular DNAs that include recognition sites for two different target genes, complementary sequences for two different Mg2+-dependent DNAzyme sequences and two different functional hairpin structures, the dendritic RCA-stimulated multiplexed analysis of two different genes is demonstrated. The amplified dendritic RCA detection of DNA is further implemented to yield the hemin/G-quadruplex horseradish peroxidase (HRP)-mimicking DNAzyme as catalytic labels that provide colorimetric or chemiluminescent readout signals.

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result of the DNA recognition event.8 Also, DNA machineries consisting of a DNA template, on which the recognition of the target-DNA triggers-on, in the presence of polymerase/dNTPs and a nicking enzyme, the isothermal autonomous synthesis of the hemin/G-quadruplex HRP-mimicking DNAzyme or RNAcleaving DNAzyme, were reported as highly sensitive optical (fluorescent, colorimetric, or chemiluminescence) sensing platforms.9 In contrast to the different amplifying schemes that involve the autonomous synthesis of catalytic labels as a result of the DNA sensing event, an alternative approach that includes the biocatalytic regeneration of the analyte was developed.10,11 For example, by using exonuclease III, Exo III, or endonucleases, the recognition complexes were cleaved, while regenerating the target-analyte for subsequent sensing events. A further goal in DNA sensing involves the multiplexed analysis of several targets. Semiconductor quantum dots (QDs)12−14 and micrometer-long metal barcode rods15 were applied for the multiplexed analysis of DNA using different readout mechanisms, such as electrical,16 fluorescence,17 Raman

he amplified detection of DNA is a continuous challenge in DNA bioanalysis. 1 Different catalysts, such as enzymes,2 catalytic nucleic acids (DNAzymes),3 or metal nanoparticles (NPs),4 were used for the amplified detection of DNA. Amplified DNA detection was accomplished by an autocatalytic and catabolic DNAzyme-mediated process.5 DNA sensing platforms of enhanced sensitivities were designed by the triggering of isothermal autonomous reactions that synthesize numerous DNAzyme units as a result of a single DNA-analyte recognition event.6−9 For example, by the design of a circular DNA scaffold consisting of the replica sequence of the hemin/G-quadruplex, the recognition of the analyte-DNA triggered-on, in the presence of polymerase/dNTPs, the rolling circle amplification (RCA) that generated DNA nanochains of the hemin/G-quadruplex horseradish peroxidase (HRP)mimicking DNAzyme enabled the colorimetric or chemiluminescence detection of DNA.6 Also, by using two functional nucleic-acid hairpin structures, the recognition of the analyteDNA by one of the DNA hairpins triggered-on the hybridization chain reaction (HCR) that led to DNAzyme chains consisting of the Mg2+-dependent DNAzyme or the hemin/Gquadruplex HRP-mimicking DNAzyme.7 A further approach for amplifying the DNA detection has included the use of the Zn2+-dependent ligation DNAzyme and the isothermal autonomous synthesis of the ligation DNAzyme units, as a © 2013 American Chemical Society

Received: October 10, 2013 Accepted: December 30, 2013 Published: December 30, 2013 1614

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Figure 1. (A) Schematic analysis of the target (1) by the dendritic RCA-stimulated synthesis of DNA chains and the amplified generation of the Mg2+-dependent DNAzyme units. The fluorescence generated by the DNAzyme-fragmented substrate provides the readout signal for the sensing process. (B) Time-dependent fluorescence changes at λem = 605 nm upon analyzing different concentrations of the analyte (1) according to the scheme shown in (A): (a) 0, (b) 1 × 10−18, (c) 1 × 10−17, (d) 1 × 10−16, (e) 1 × 10−15, (f) 1 × 10−14, (g) 1 × 10−13, (h) 1 × 10−12, (i) 1 × 10−11, (j) 1 × 10−10, (k) 1 × 10−9, (l) 1 × 10−8, and (m) 1 × 10−7 M. (C) Fluorescence spectra generated by the system upon analyzing different concentrations of the target (1) for a fixed time interval of 3 h. (a) to (g) concentrations as in (B). Inset: Derived calibration curve corresponding to the fluorescence intensities at λem = 605 nm upon analyzing different concentrations of the target (1).

spectroscopic fingerprints,18 fluorescence resonance energy transfer (FRET),13 and chemiluminescence resonance energy transfer (CRET)14 processes. Similarly, the selective desorption of different probes labeled with different fluorophores from graphene oxide supports through the selective formation of probe/analyte duplexes was used for the multiplexed analysis of DNA.19 In the present study, we implement the isothermal rolling circle amplification (RCA) process to trigger a dendritic RCAinduced formation of amplifying DNAzyme catalytic labels. We describe the dendritic RCA-mediated formation of the Mg2+dependent DNAzyme or of the hemin/G-quadruplex HRPmimicking DNAzyme units and the use of fluorescence, color, or chemiluminescence as readout signals for the different sensing platforms. Finally, by applying two different circular DNA templates, the multiplexed analysis of two different genes is demonstrated with the parallel detection of the Tay-Sachs

genetic disorder mutant and the gene associated with the TP53 pathogen.

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EXPERIMENTAL SECTION

Materials. Phi29 DNA polymerase and deoxyribonucleoside 5′-triphosphate mixture (dNTPs), Escherichia coli Exonuclease I (Exo I), Exonuclease III (Exo III), T4 polynucleotide kinase, and Quick Ligation Kit were obtained from New England BioLabs, Inc. Luminol and H2O2 were purchased from Sigma. Hemin was purchased from Frontier Scientific, Inc. A hemin stock solution (1.0 mM) was prepared in DMSO and stored in the dark at −20 °C. All DNA oligonucleotides were purchased from Integrated DNA Technologies Inc. (Coralville, IA). Table S1 of the Supporting Information depicts the sequences of the oligonucleotides used in the present study. The oligonucleotides were HPLC-purified and dissolved in phosphate buffer (10 mM, pH = 7.0) to yield stock solutions of 100 μM. 1615

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analyte (1) according to Figure S3(A) of the Supporting Information and running the RCA amplification process for a fixed time interval of 3 h. As the concentration of the analyte is higher, the resulting fluorescence is intensified, consistent with the higher content of RCA-generated Mg2+-dependent DNAzyme units. Figure S3(C) inset of the Supporting Information shows the resulting calibration curve corresponding to the fluorescence intensities of ROX (F1) at λem = 605 nm upon analyzing different concentrations of the target-DNA (1), implying that the analyte could be analyzed with a detection limit that corresponds to 1 pM. Figure 1A depicts the dendritic RCA detection of the analyte [the Tay-Sachs genetic disorder mutant (1)] using the Mg2+dependent DNAzyme as catalytic reporter and fluorescence as the readout signal. The system consists of the circular DNA (2) that includes three functional sequences (I, the recognition sequence for the analyte; II, the complementary sequence for the dendritic branching element of the RCA reactions; and III, the complementary sequence of the Mg2+-dependent DNAzyme), the fluorophore/quencher (F1/Q1)-functionalized substrate (3) of the DNAzyme, and the hairpin structure (4), acting as key-element for the dendritic RCA reaction. The hairpin structure (4) is composed of a stem region that includes the analyte sequence I′ [complementary to the sequence I of circular DNA (2)] and a loop region comprising the sequence II, present in the circular DNA (2). Under these conditions, the sequence I′ of the hairpin is blocked in an inactive structure. In the presence of the analyte (1) and the polymerase/dNTPs mixture, the RCA process is triggered-on the circular DNA template (2), leading to the RCA chains composed of the I′− III′−II′ repeat units. The component II′ associated with the nanochains is complementary to the single-stranded loop II associated with the hairpin structure (4). This results in the hybridization of the sequences II′ of the RCA chains with the loop region of the hairpin (4), leading to the opening of the hairpin and the release of the free sequence of the analyte, domain I′, that was originally caged in the stem region of the hairpin. The resulting released sequence I′ binds to the circular DNA template (2), resulting in the first generation of branched RCA chains comprising the I′−III′−II′ repeat units. That is, the primary recognition of the analyte (1) by the circular template (2) triggers the isothermal autonomous dendritic formation of branched RCA chains consisting of the Mg2+-dependent DNAzyme sequences III′. The hybridization of the substrate (3) with the DNAzyme sequences triggers-on the cleavage of the substrate and the generation of fluorescence (F1 = ROX, λem = 605 nm). Figure 1B shows the time-dependent fluorescence changes at λem = 605 nm upon analyzing different concentrations of the analyte (1) by the system outlined in Figure 1A. A nonlinear increase in the fluorescence intensities is observed. Interestingly, the intense fluorescence changes includes an induction time interval after which a sharp increase in the fluorescence is observed. As the concentration of the analyte (1) is lower, the induction time interval is longer, and the onset in the fluorescence increase appears after a longer time interval. These results are consistent with the nonlinear dendritic RCA formation of the Mg2+-dependent DNAzyme units. The lower the concentration of the target (1), the longer is the time interval needed for the generation of this threshold value. For optimal sensitivity and S/N > 3, we analyze the target DNA (1) after a fixed time interval of 3 h. This time interval was selected due to the observation that the background fluorescence signal of the system in the absence

Ultrapure water from a NANOpure Diamond (Barnstead) source was used in all of the experiments. RCA Assays. The preparation of the circular DNAs, (2), (6), and (9), is detailed in Figures S1 and S2 and the accompanying discussions of the Supporting Information. In all systems, the fixed concentrations of the hairpin (0.2 μM, for dendritic RCA only), the circular DNA (0.2 μM), the fluorophore/quencher-modified DNAzyme substrates (1.0 μM, for Mg2+-dependent DNAzyme only) were employed. Phi29 DNA polymerase (0.5 units/μL), dNTPs (0.2 mM), and different concentrations of the target DNA were included to initiate the RCA reaction. The RCA process was performed in a buffer solution consisting of 50 mM Tris-HCl, pH 7.5, 20 mM MgCl2, and 1 mM DTT. Fluorescence measurements were performed using a Cary Eclipse spectrometer (Varian inc). For the readout of the dendritic RCA synthesis of the hemin/Gquadruplex HRP-mimicking DNAzyme, colorimetric measurements were performed in a solution consisting of the products, hemin (0.1 μM), H2O2 (44 μM), and ABTS2− (0.18 mM), in a 10 mM HEPES buffer solution (pH 7.4, 200 mM NaCl). Absorbance changes were followed at λ = 420 nm to characterize the rate of the oxidation of ABTS2− to ABTS−• by H2O2. Chemiluminescence measurements were performed using a photon counting spectrometer (Edinburgh Instruments, FLS 920) equipped with a cooled photomultiplier detection system, connected to a computer (F900 v.6.3 software). Measurements were made in a 10 mM HEPES buffer solution (pH 8.5, 200 mM NaCl), which included the product, hemin (0.1 μM), luminol (0.5 mM), and H2O2 (30 mM).

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RESULTS AND DISCUSSION The basic rolling circle amplified (RCA) detection of the analyte-DNA, the Tay-Sachs genetic disorder mutant (1), is depicted schematically in Figure S3(A) of the Supporting Information. The sensing system includes the circular DNA (2) composed of the domain I (complementary to the analyte sequence I′), the region II, and the sequence III that is complementary to the Mg2+-dependent RNA-cleaving DNAzyme sequence. Also, the ribonucleobase (rA)-containing nucleic acid (3), modified by a fluorophore/quencher (F1/ Q1) pair (where F1 = 6-carboxyl-X-rhodamine, ROX; Q1 = Black Hole Quencher-2), acts as the substrate of the Mg2+dependent DNAzyme. In the presence of the analyte (1), polymerase and the dNTPs mixture, the RCA process is triggered-on, leading to the isothermal autonomous synthesis of the RCA chains composed of the repeat units I′−III′−II′ (where domain III′ is composed of the Mg2+-dependent DNAzyme sequence). The substrate (3) is modified at its 5′and 3′-ends with a fluorophore/quencher (F1/Q1) pair, respectively, resulting in the quenching of the luminescence of the fluorophore (F1 = ROX, λem = 605 nm). The formation of the DNAzyme chains triggers the cleavage of the substrate (3), leading to a fluorescence signal, and this provides the readout signal for the analysis of the target-DNA (1). Figure S3(B) of the Supporting Information shows the timedependent fluorescence changes at λem = 605 nm upon analyzing different concentrations of the analyte (1) by the system outlined in Figure S3(A) of the Supporting Information. Nonlinear increase in the time-dependent fluorescence changes is observed, consistent with the increase in the content of DNAzyme reporter units as the RCA process proceeds. Figure S3(C) of the Supporting Information shows the fluorescence spectra observed upon analyzing different concentrations of the 1616

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Figure 2. Fluorescence spectra generated by the dendritic RCA system after a fixed time interval of 3 h upon analyzing: (A) no target; (B) target (1) only, 10 nM; (C) target (5) only, 10 nM; (D) targets (1) and (5), 10 nM each. Curves a and b correspond to the fluorescence of ROX (F1) and FAM (F2), respectively.

the different substrates with different fluorophore/quencher units, diverse catalytic DNAzyme labels for multiplexed analyses may be envisaged. That is, by encoding in the base sequence of the circular DNA template the information to synthesize different Mg2+-dependent DNAzymes, the multiplexed, parallel analysis of different targets may be achieved. Accordingly, we designed a second circular DNA (6), and a new functional hairpin structure (8) for the dendritic RCAstimulated synthesis of the Mg2+-dependent DNAzyme nanowires for the amplified detection of a second target, the TP53 gene (5) (see Figure S4 of the Supporting Information). The circular DNA template (6) includes the recognition sequence (domain V) for the target gene (5), the sequence (domain VI) that provides the initiator unit for the dendritic branching mechanism, and the domain VII that consists of the sequence complementary to the tailored Mg2+-dependent DNAzyme. Also, the hairpin structure (8) and the F2/Q2-labeled substrate (7) (where F2 = 6-carboxyfluorescein, FAM; Q2 = Black Hole Quencher-1) are included in the system. The hairpin structure (8) includes in its single-stranded loop domain, the sequence VI and in its stem region the caged target sequence V′. In the presence of the analyte (5) and polymerase/dNTPs mixture, the RCA process over the template (6) proceeds to yield the DNA chains composed of the repeat units V′−VII′−VI′. The autonomous opening of the hairpin units (8) by the sequences VI′ in the RCA-generated nanochains and the association of the hairpin-released target domains V′ to further circular template units (6) triggers the dendritic formation of branched RCA chains that include the tailored Mg2+-dependent DNAzyme sequences for cleaving the substrate (7) and releasing the F2labeled fragmented nucleic acid (F2 = FAM, λem = 520 nm) as a fluorescence readout signal for analyzing the target (5). This enables the amplified detection of the target gene (5) with a detection limit corresponding to 1 aM. (For the detailed detection of the target gene (5) by the regular RCA process generating the Mg2+-dependent DNAzyme units and by the

of the analyte is almost zero for a time interval of 3 h, and after this time-gap, a perturbing sharp increase in the background fluorescence is observed. Note that the system includes, in the absence of the target (1), the hairpin structure (4) that includes in its stem region the caged sequence of the target. As traces of the open hairpin are always present in the system, these trace amounts of target sequences (I′) activate the dendritic RCA process and the autonomous synthesis of the Mg2+-dependent DNAzyme reporter units. Nonetheless, due to the low concentration of the open hairpins, the time interval required to reach the set-on fluorescence changes is longer than three hours. That is, the analysis of the target DNA by the dendritic RCA machinery is unperturbed for a time interval of three hours. Figure 1C shows the resulting fluorescence spectra corresponding to the analysis of different concentrations of the target DNA (1) by the system outlined in Figure 1A for a fixed time interval of 3 h. Figure 1C inset shows the resulting calibration curve corresponding to the fluorescence intensities of ROX at λ em = 605 nm upon analyzing different concentrations of the target DNA (1). The detection limit for analyzing the Tay-Sachs genetic disorder mutant (1) by the dendritic RCA synthesis of the Mg2+-dependent DNAzyme units is 1 aM. The use of the Mg2+-dependent DNAzyme as a reporter unit for the sensing event enables the design of multiplexed dendritic RCA amplification schemes. The Mg2+-dependent DNAzyme requires a conserved nucleic acid sequence for the binding of the Mg2+-ion and a conserved base-sequence containing a ribonucleobase (rA) that acts as substrate for the DNAzyme and hybridizes with the complementary bases associated with the DNAzyme sequence. The tethering of tandem base sequences at the 3′- and 5′-ends of the conserved DNAzyme sequences and the elongation of the respective 5′and 3′-ends of the substrates with complementary tandem base sequences allows the tailoring of diverse Mg2+-dependent DNAzymes that bind different substrates. Thus, by modifying 1617

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Figure 3. (A) Schematic analysis of the target (1) by the RCA-stimulated synthesis of DNA chains that trigger the opening of hairpin (4) and the dendritic synthesis chains consisting of the hemin/G-quadruplex HRP-mimicking DNAzyme units. The DNAzyme catalyzes the H2O2-mediated oxidation of ABTS2− to the colored product ABTS−• or the H2O2-mediated oxidation of luminol to yield chemiluminescence. (B) Time-dependent absorbance changes, following ABTS−• at λ = 420 nm, upon analyzing different concentrations of the target (1), using the sensing platform shown in (A): (a) 0, (b) 1 × 10−18, (c) 1 × 10−17, (d) 1 × 10−16, (e) 1 × 10−15, (f) 1 × 10−14, (g) 1 × 10−13, (h) 1 × 10−12, (i) 1 × 10−11, (j) 1 × 10−10, (k) 1 × 10−9, (l) 1 × 10−8, (m) 1 × 10−7 M, and (n) hemin only. Inset: Calibration curve corresponding to the absorbance changes observed after a fixed time interval of 3 h upon analyzing different concentrations of the target (1). (C) Chemiluminescence spectra corresponding to the analysis of different concentrations of the target (1) according to the scheme outlined in (A). (a−n) concentrations as in (B). Inset: Calibration curve corresponding to the chemiluminescence intensities at λ = 420 nm upon analyzing different concentrations of the target DNA (1). The dendritic RCA reactions were terminated after a fixed time interval of 3 h for all the systems.

dendritic RCA-stimulated synthesis of the Mg2+-dependent DNA units, see Figure S5 and S4 of the Supporting Information.) We then used a mixture consisting of the circular DNA (2) and the hairpin (4) pair with the circular DNA (6) and the hairpin (8) pair for the multiplexed analysis of the two targets (1) or/and (5) using (3) and (7) as substrates for the Mg2+-dependent DNAzymes generated by the respective circular DNA/hairpin pairs. The resulting fluorescence of the fragmented substrates F1 and/or F2 (F1 = ROX, λem = 605 nm; F2 = FAM, λem = 520 nm) provides then the readout signal for the respective analyte. The multiplexed analysis of the targets (1) and/or (5) is shown in Figure 2. In this study, we present the analysis of the two targets, 10 nM each, by the dendritic RCA mechanism using the Mg2+-dependent DNAzymegeneration of the fluorophore-labeled DNA fragments as

readout signals. Figure S6 of the Supporting Information shows the time-dependent fluorescence changes upon analyzing each of the targets (1) or/and (5) by the composite mixture. Figure 2 shows the fluorescence spectra of the system generated after a fixed time interval of 3 h upon subjecting the analyzing mixture to no target, Figure 2A; target (1) only, Figure 2B; target (5) only, Figure 2C; and upon subjecting the mixture to the two targets (1) and (5) together, Figure 2D. The fluorescence of F1 (ROX, λem = 605 nm) is triggered-on upon analyzing the analyte (1), whereas the fluorescence of F2 (FAM, λem = 520 nm) is activated in the presence of the analyte (5). In the presence of the two targets, the fluorescence of F1 and F2 are activated, and thus, the multiplexed analysis of the two targets is demonstrated. 1618

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difficult. For the comparison of the present amplification sensing platform to other isothermal DNA amplification systems, see Table S2 of the Supporting Information. For a comparison corresponding to the specific sensing of the TaySachs gene by the present sensing platform and other reported systems, see Table S3 of the Supporting Information. The information included in Tables S2 and S3 of the Supporting Information indicates that the present method reveals superior analytical performance for analyzing DNA, and specifically, for the detection of the Tay-Sachs gene. In order to examine the implementation of the amplified sensing platform of DNA in “real” bioenvironments, we examined the detection of the target-DNA (1) in plasma samples using the dendritic RCA process that leads to the formation of the hemin/G-quadruplex DNAzyme according to Figure 3A. Toward this end, variable concentrations of the target (1) were dissolved in plasma, the samples were diluted to 5% and subjected to the dendritic RCA analyses using the hemin/G-quadruplex DNAzyme-catalyzed oxidation of ABTS2− to the colored product ABTS−• by H2O2 as the readout signal. Figure 4 shows the time-dependent absorbance changes of

The use of other DNAzymes as catalytic reporter units for the dendritic RCA process enables the implementation of DNAzymes with high turnover numbers and the use of DNAzyme that produce readout signals other than fluorescence. This was demonstrated by the implementation of the hemin/G-quadruplex HRP-mimicking DNAzyme.20 This DNAzyme allows the catalyzed colorimetric21 or chemiluminescence14,22 transduction of the sensing events. Accordingly, the dendritic RCA detection of target (1) was achieved by applying the hemin/G-quadruplex HRP-mimicking DNAzyme as a catalytic label for the generation of the color or chemiluminescence readout signals via the DNAzyme-catalyzed oxidation of 2,2′-azino-bis-(3-ethylbenzothialine)-6-sulfonic acid (ABTS2−) to the colored product ABTS−• by H2O2 or the DNAzyme-catalyzed oxidation of luminol by H2O2, respectively. The system included a circular DNA (9) as a template and the hairpin structure (4) as a functional unit that triggers the dendritic RCA process (see Figure 3A). The circular DNA (9) is composed of the sequence I that is complementary to the target (1), the sequence II provides the branching element for the dendritic RCA, and the domain IV consists of the sequence complementary to the hemin/Gquadruplex DNAzyme sequence. In the presence of the analyte (1) and the polymerase/dNTPs mixture, the RCA process is initiated to yield the RCA chain consisting of the repeat units I′−IV′−II′. The component II′ in the RCA chains opens hairpin (4), leading to the release of the free analyte sequence I′. The uncaging of the analyte sequence and its hybridization with the circular template units (9), results in the dendritic branching of the RCA process and the amplified detection of the target (1). The dendritic RCA-generated chains include the domain IV′ corresponding to the G-quadruplex sequence. As a result, the branched hemin/G-quadruplex DNAzyme units are formed, and these catalyze the formation of the color or chemiluminescence as readout signals. Figure 3B depicts the time-dependent absorbance changes as a result of the hemin/ G-quadruplex DNAzyme-catalyzed oxidation of ABTS2− by H2O2 to yield the colored product ABTS−• (λmax = 420 nm), according to the system outlined in Figure 3A. As the concentration of the analyte (1) increases, the absorbance changes are higher. Figure 3B, inset, depicts the calibration curve corresponding to the absorbance changes generated by the dendritic RCA detection system after a fixed time interval of 3 h upon analyzing different concentrations of the target DNA (1). The detection limit for analyzing the target DNA (1) by the RCA-stimulated dendritic synthesis of the hemin/Gquadruplex DNAzyme and the colorimetric readout signal corresponds to 1 aM. (For the detailed detection of the target gene (1) by the regular RCA process generating the hemin/Gquadruplex DNAzyme units, see Figure S7 of the Supporting Information.) Similarly, Figure 3C shows the chemiluminescence spectra generated by the hemin/G-quadruplex DNAzyme formed by the dendritic RCA process outlined in Figure 3A for a fixed time interval of 3 h and through the DNAzymecatalyzed oxidation of luminol by H2O2. Figure 3C, inset, shows the resulting calibration curve relating the chemiluminescence intensity at λ = 420 nm generated by the dendritic RCA detection system, to variable concentrations of the target (1). The resulting detection limit is comparable to the detection limit observed in the colorimetric detection system, 1 aM. The disadvantage of the system consisting of the dendritic RCA synthesis of the hemin/G-quadruplex HRP-mimicking DNAzyme is, however, the fact that multiplexed analyses of targets is

Figure 4. Time-dependent absorbance changes, following ABTS−• at λ = 420 nm, upon analyzing different concentrations of the target (1) in plasma, using the sensing platform shown in Figure 3A: (a) 0, (b) 5 × 10−15, (c) 1 × 10−13, (d) 2 × 10−12, (e) 1 × 10−10, and (f) 1 × 10−8 M. Inset: Calibration curve corresponding to the absorbance changes observed after a fixed time interval of 3 h upon analyzing different concentrations of the target (1).

ABTS−• upon analyzing different concentrations of (1) in the plasma. The results imply that the target (1) could be sensed with a detection limit of 5.0 fM in the plasma samples, with a reproducibility of ±5% (N = 5 experiments).

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CONCLUSIONS The present study has extended the isothermal rolling circle amplification (RCA) process to a RCA-stimulated dendritic synthesis of catalytic nucleic acids (DNAzymes) sensing platform. The method is based on the improvement of the RCA sensing methods by two elements: (i) The introduction of a functional hairpin that regenerates the analyte and allows the dendritic branching of the RCA chains. (ii) The isothermal autonomous formation of DNAzymes in the RCA chains. These catalytic labels enabled the fluorescence, colorimetric, or chemiluminescence detection of the analytes. By the tailoring of different circular DNA templates that lead to the formation of different RCA-branched DNAzyme reporter catalysts, the 1619

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multiplexed analysis of different analytes was demonstrated. The isothermal autonomous RCA-stimulated dendritic generation of DNAzymes enabled the ultrasensitive detection of the analyte.

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ASSOCIATED CONTENT

S Supporting Information *

Normal RCA detection of Tay-Sachs mutant gene, Normal and dendritic RCA detection of TP53, kinetics of the multiplexed detection system, synthesis and characterization of the circular DNA templates. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 972-2-6585272. Fax: 9722-6527715. Author Contributions ‡

F.W. and C.-H.L. contributed equally to the research. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Parts of this research are supported by the Israel Science Foundation and by the Volkswagen Foundation, Germany.

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REFERENCES

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