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In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/chemcomm Page 1 of 4 Journal Name ChemComm Dynamic Article Links ► Cite this: DOI: 10.1039/c0xx00000x www.rsc.org/xxxxxx ARTICLE TYPE A tailor-made nucleoside-based colourimetric probe of formic acid Julie P. Vanegas ,a Elena. Zaballos-García, ,b and Julia Pérez Prieto a,* Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x 5 A ratiometric, specific probe of formic acid has been on the basis of a demanding unzipping of the nucleoside channels developed. It is based on intermolecular nucleobase-pairing of of inosine-capped AuNP (Au@S-IN) aggregates (Fig. 1). inosine-capped plasmonic nanoparticles to form nucleoside Au Au@S-IN channels, which are destabilised by the analyte. N N 1 N There are a considerable number of specific probes of ions, but N N 10 N 0.75 this rarely applies to small organic molecules. Gold nanoparticles N 50 N (AuNPs) are of particular interest in colourimetric sensing. On O H H O O 0.5 the one hand, the large surface-to-volume ratio of spherical NPs O H H N 6 6 Absorbance can be used advantageously for placing a considerable number of N 1 0.25 1 N 3 7 N Manuscript specific functionalities on their periphery, i.e., it makes it possible 7 3 N N N 0 15 to have a high local concentration of a functional group in an N 300 400 500 600 700 800 Wavelength/nm otherwise very diluted solution. On the other hand, nanometre- Au@S-IN sized AuNPs display intense colour, thus being clearly detectable Au by the naked eye at nanomolar concentrations, and, in addition, Fig.1: Left: Schematic representation of the formation of Au@S-IN aggregates by 55 hydrogen-bonding. Right: UV-Visible absorption spectrum of Au@S-IN aggregates they exhibit strong distance-dependent optical properties. Thus, in chloroform 20 in the aggregated state, they form blue colloidal solutions, which We demonstrate here that Au@S-IN aggregates possess an change to intense burgundy (a typical plasmonic band at λmax ca. exceptional capacity for the specific sensing of formic acid. The 1,2,3 530 nm) when the NPs are well-dispersed. Consequently, aggregates remain stable in the presence of methanol, ethanol, AuNPs are being applied as colourimetric probes by making use 60 and alcohols of a higher weight, as well as in the presence of Accepted of analyte-induced changes in their aggregation state. Usually, other carboxylic acids (even acetic acid or more acidic 25 large biomolecules, such as oligonucleotides, DNA, and proteins compounds, such as chloroacetic and trifluoroacetic acids). at the NP periphery are used to obtain systems that are responsive to small molecules, which induce the assembly of the Theoretical calculations 15 have shown that adenine-capped NPs. 4,5,6,7,8,9,10,11, 12,13 ,14 Alternatively, aggregated NPs can be AuNPs (Au@S-AD) give rise to nucleoside channels in low polar disassembled by metal ions or other analytes and the colour of the 65 solvents due to the supramolecular assembly of the AuNPs by .7 30 colloid gradually changes from blue to red. means of hydrogen bonds (HBs) between the adenine moieties belonging to different nanoparticles. The sensing selectivity to Probes based on disassembly of metal NPs capped with small small alcohols, such as methanol and ethanol, has been attributed ligands are uncommon. 15 In addition, it is hard to design to the penetration of the small alcohols into the nucleoside colourimetric probes exhibiting selective molecular recognition 70 channels created in the structure, combined with the anchoring of of small molecules (one or two carbon atoms) from compounds the alcohol to the N7 basic centre of the nucleobase; this centre enhances the methanol basicity. Consequently, the molecular 35 of a higher molecular weight of the same chemical family. We ChemComm have recently reported a colourimetric method for sensing of recognition capacity of the Au@S-AD hybrids has been ascribed methanol and ethanol in an organic media based on AuNPs to size exclusion effects combined with “allosteric” weakening of 75 the inter-nanoparticle HBs. Taking into account the variety of decorated with adenosine (AD) derivatives (Au@S-AD). 15 nucleobases available, this strategy could be expanded to detect Au@S-AD aggregates were insensitive to n-propanol, i-propanol, selectively other analytes by making use of the particular features 40 n-butanol, and t-butanol. of the nucleobases in respect to i) the relative position of their hydrogen donor/acceptor centres, ii) the HB strength of their These unprecedented results prompted us to explore the 80 nucleobase-nucleobase pairing, and/or iii) the size of the resulting usefulness of the particular features of the inosine (IN) channels after Au@nucleobase aggregation. nucleobase, specifically its extraordinary capacity for self- Inosine, which is commonly found in tRNAs and is essential for association in low-polar solvents (690 ± 100 M −1 in 16 proper translation of the genetic code in wobble base pairs, is a 45 chloroform), to build up a highly selective probe of formic acid This journal is © The Royal Society of Chemistry [year] [journal] , [year], [vol] , 00–00 | 1 ChemComm Page 2 of 4 nucleoside comprising hypoxanthine attached to a ribofuranose 55 disassembly of the NPs, evidenced by the blue-shift of the via a β-N9-glycosidic bond. 17 IN self-associates in chloroform, absorption band up to ca. 530 nm with a concomitant narrowing but the associates differ from those of AD in two key ways: i) AD of the spectrum (Fig. 2B). Interestingly, colour changes occurred can coexist in a variety of isomeric forms, but IN exists in only rapidly and, therefore, each UV/Vis spectrum was registered at 20 16 5 one and ii) the IN association constant (K) in chloroform is ca. 1 minute after the addition of each formic aliquot. considerably greater than that of AD 18,19 (690 ± 100 M −1 and 1.6 −1 ±0.2 M , respectively). Consequently, Au@S-IN nanohybrids 60 A would form stronger aggregates in chloroform than Au@S-AD hybrids and, therefore, they would be considerably more difficult 10 to disrupt. To test our hypothesis, first we prepared oleylamine-stabilized AuNPs (Au@OA), with an average core diameter of 10.0±0.4 nm. These AuNPs gave rise to burgundy-coloured solutions in chloroform and their UV-Vis absorption spectrum exhibited the 1 B C 15 typical plasmon band at λmax 525 nm. Au@OA nanoparticles 65 HCOOH 1.0 reacted with 5´-(11-mercaptoundecamido)-5´-deoxy-2´,3´-O- 0.8 Acetic Acid isopropylideneinosine (HS-IN) to give rise to Au@S-IN 0.6 530 Formic acid /A nanoparticles (see all the experimental details and spectra of 0.4 700 A 0.5 Au@OA , HS-IN, Fig. S1-S4, in the supporting information). The Absorbance 0.2 20 red-shift of the plasmon peak, accompanied by a considerable 0 broadening of the spectrum (Fig. 1), evidenced the decrease in the 300 400 500 600 700 800 0 2 4 6 8 interparticle distance and suggested HB interactions between the 70 Wavelength/nm [ mol/L] Fig. 2. A) pictorial representation of the Au@S-IN aggregates and action of formic Manuscript nucleobase moieties located around the periphery of different acid on the IN nucleobase thereby causing unzipping of the nucleoside channels; B) NPs (Fig. 1). The aggregates, which are typically blue, proved UV-Visible absorption spectra of Au@S-IN aggregates in chloroform in the absence 12 and in the presence of formic acid; the arrows indicate the absorbance trend with 25 stable in chloroform. As previously reported for IN , such 75 increasing [HCOOH]; C) Plots of the A 700 /A 530 ratio vs [HCO 2H] and [H 3CCO 2H] for interaction would involve the NH1---O=C6 hydrogen bonding Au@S-IN aggregates in chloroform. between the IN units. The alignment of such units, belonging to different nanohybrids, would induce an antiparallel arrangement A reliable method to analyse the capacity of an analyte to induce between the nanoparticles and create a series of nucleoside the disassembly is by monitoring the ratio between the 80 absorbance at 700 nm and that of the NP plasmon peak maximum 30 channels (Fig. 1). This would permit the movement of molecules of the right size within the channels. The disassembly of the (ca. 530 nm); a low ratio is associated with dispersed red NPs and a high ratio is associated with blue aggregates. Ratiometric Au@S-IN aggregates would be induced through destabilisation of Accepted the inter-particle HBs by increasing the concentration of the probes are suitable for practical applications since they are less appropriate analyte (Fig.