High Extinction Coefficient Research Articles

Electrodeposition of Plasmonic Nanostructures

Electrochemical control and the use of electrodeposition in the design of dynamic plasmonics have attracted much attention in recent years.1 Development of dynamic plasmonic metamaterials is attractive for many applications such as molecular sensing and analysis, environmental monitoring, photo-catalysis, colour changing displays and electrochromic devices such as 'smart' windows. Electrodeposition is one of the most attractive ways to create and reversibly transform nanostructures' shape, size and chemical composition.2,3 Plasmonics is related to the localised surface excitations of electrons in metal nanostructures due to strong interactions with light. The resulted electric field enhancement due to the surface plasmons can be used to manipulate light–matter. Nanostructured Ag and Au are classic plasmonic materials. While silver is a metal that exhibits many advantages over gold, such as higher extinction coefficients in the blue and UV region of the EM spectrum, sharper extinction bands and extremely high field enhancements, its employment is hindered by low chemical stability. The most recent theoretical analysis suggests that Au-Ag derived nanostructures with controlled geometry, composition, and distribution can create new interesting optical phenomena.4 By developing Au-Ag based nanostructures, we can then benefit from combining optical properties of Au and Ag and, at the same time, improve the chemical stability of silver.We investigated the electrodeposition of Au and Ag-based arrays of ordered and random nano-particles on indium tin oxide substrates from different solutions and studied their optical properties. We demonstrated that varying the electrodeposition parameters led to changes in both the resonance wavelength and the strength of resonance linked to the structural characteristics (size and shape) and the chemical composition of the deposited particles. Exploration of the dynamic reversible changes via electrodeposition will be presented.

New Degradable Semiconducting Polymers for Photoacoustic Imaging of λ-Carrageenan-Induced Arthritis Mouse Model.

Semiconducting polymer has a high extinction coefficient and a long band absorption and can be used as a photoacoustic imaging contrast agent. However, nonbiodegradable semiconducting polymers may cause biosafety issues due to being retained in the body. Therefore, developing degradable semiconducting polymers is necessary for in vivo imaging. Herein, we developed three degradable semiconducting polymers with unique optical properties. We adjusted the optical properties of semiconducting polymers by designing the molecular structure of semiconducting polymers. Polymers with a donor-π-acceptor structure could easily improve the optical properties through adjusting the donor or acceptor units. Through adjusting the electron-donor and -acceptor units, three diketopyrrolopyrrole derivative polymers (DPPTz, DPPQu, and DPPWu) were synthesized and converted into nanosize particles. By introducing the degradable chemical groups in the main chain structure of semiconducting polymers, diketopyrrolopyrrole polymers could be degraded by ClO-. Among these nanosize particles, DPPTz NPs and DPPQu NPs were used to achieve the in vivo photoacoustic imaging of λ-carrageenan-induced arthritis mouse model. This work provides a novel design idea for the designing of red-shifted semiconducting polymer with degradable properties.

Effect of TiO<sub>2</sub> Nanoparticles with High Light Absorption on Improving Growth Parameters and Enzymatic Properties of Sorghum <i>(Sorghum bicolor L. Moench)</i>

AbstractTitanium dioxide nanoparticles are great boosters for better sunlight absorption by plants. However, their band gap is so wide that they can only absorb UV light, which is a small portion of the sunlight. To improve the absorption of visible light, in this work, titanium nanoparticles were sensitized by saffron dye and the effect of foliar application of the dyed nanoparticles on growth parameters and enzymatic properties of sorghum was investigated and the results were compared with those of conventional titanium dioxide nanoparticles. The experiment was conducted in Zabol region, Iran. Saffron, a natural organic dye, was used as a sensitizer because of its availability and high extinction coefficient in the visible regions. To perform sensitization, titanium dioxide nanoparticles were immersed in saffron dye solution to absorb the dye on their surface. Then different concentrations of the sensitized nanoparticles were applied on plants by foliar spraying. The most improved growth parameters including root and shoot lengths, shoot fresh and dry weights, root fresh and dry weights were obtained after application of dye-sensitized TiO2 nanoparticles. Zinc and iron are the elements highly affected by the application of new nanoparticles. Enzyme activities of catalase, ascorbate peroxidase and guaiacol peroxidase were also increased significantly. Chlorophyll a and Chlorophyll a/b ratio showed the highest values in 500 ppm dye-sensitized TiO2 nanoparticles. The obtained results confirm the improvement in growth parameters and enzymatic properties of Sorghum after application of the new TiO2 nanoparticles.

Fluorescence enhancement of near infrared cell membrane probe by β-cyclodextrin supramolecular interaction

In this paper, oxazolepyridine and coumarin units were combined to prepare cell membrane fluorescent probe (PM-A) through aldol condensation reaction. PM-A had long wavelength emission (665–739 nm), high molar extinction coefficient (around 3.14 × 104 M−1cm−1) and large Stokes shift (150–219 nm). The photo-stability test showed that PM-A had a high relative residual absorption (93%) after 6 h of continuous illumination. In addition, the supramolecular interaction between PM-A and β-cyclodextrin was evaluated and applied to cell imaging. Probe PM-A had no fluorescence in water, but had obvious fluorescence signal in β-cyclodextrin solution. With the addition of β-cyclodextrin (0–800 eq), the fluorescence quantum yield of PM-A increased from 0% to 45%. Laser confocal experiments showed that when only PM-A was added, the probe concentration needed to be 15 μM to image the cell membrane obviously. In sharp contrast, with the help of β-cyclodextrin solution, the concentration of probe PM-A only needed 0.5 μM to quick and high specificity image cell membranes. Moreover, PM-A had high photo-bleaching resistance and long retention time after adding β-cyclodextrin. These excellent properties of PM-A were of great significance to the study of cell membrane.

Environmentally compatible 3‐dimensional star‐shaped donor materials for efficient organic solar cells

Small molecule (SM)-based star-shaped materials are getting tremendous attention to be employed as hole transport materials for organic/perovskite solar cells (OSCs) due to their tunable energy levels, intense absorption ability, and their three-dimensional (3D) charge transport properties. Herein, four-armed SM donors (BD1-BD10) with benzodithiophene (BDT) as the central-core unit have been efficiently designed and then characterized theoretically to investigate their optical and optoelectronic behavior. These four-armed designed star-shaped (BD1-BD10) materials exhibited deeper HOMO levels and higher extinction coefficients, which tend to offer better phase separation morphology during blend formation. These molecules (BD1-BD10) and reference (R) have been fully characterized theoretically with advanced quantum chemical approaches. The photophysical and optoelectronic characteristics have been investigated with density functional theory (DFT) and time-dependent (TD-DFT) calculations. The alignment of frontier molecular orbitals (FMOs), optical characteristics, open-circuit voltages, the density of states (DOS), transition density matrix (TDM), and reorganization energies of holes and electrons in these materials have been investigated. BD3 shows the highest absorption (λmax) of 571.33 nm, with an optical bandgap at 3.30 eV, respectively. Moreover, a complex study of BD3/PC61BM unveiled the process of remarkable charge shifting at the donor-acceptor interface. Therefore, our proposed strategy is a prerequisite for designing desirable photovoltaic molecules for efficient organic/perovskite SCs and light-emitting diodes (LEDs).

Effect of Structural Variation on Spectral, NLO Properties, and Biological Activity of Pyrrole Hydrazones.

This paper describes spectral, non-linear optical (NLO), and biological activity variation of pyrrole hydrazones as a result of structural variation. In order to study structure–property variation, pyrrole hydrazones (3A, 3B, and 3C) were synthesized in both solid and solution phases. The grinding solvent-free method becomes rapid, easy, convenient, useful, sustainable, and eco-friendly green synthesis as compared to the classical solution phase reactions. The structure of pyrrole hydrazones has been elucidated by microanalysis and quantum chemical calculations. The intense emission at λem 521 nm (3A and 3B) and 617 nm (3C) in the visible (green and orange) region with Stokes shifts at 195, 160, and 282 nm reveals that the studied compounds work as good photoluminescent materials. All compounds show strong n−π* and charge transfer (π–π*) transitions in the UV–vis region with high extinction coefficients. In the studied systems (3A, 3B, and 3C), the orbital overlap between σ(NH–O) → σ*(NH–O) is found due to intra-molecular charge transfer. The first hyperpolarizabilities were found to be 48.83 × 10–30 esu for 3B and 63.89 × 10–30 esu for 3C, showing variation with structure. Their high values indicate more suitability for NLO application. Incorporation and/or change in position of electron-withdrawing groups increase the β0 values of 3B and 3C compared to 3A. The β value also increases monotonically as the polarity of the solvents increases. The red shift in N–H and C=O stretching Fourier-transform infrared bonds is due to the formation of dimers. The synthesized 3A, 3B, and 3C show good antimicrobial activity and are predicted to be potential antibacterial and antifungal drugs. The 3B has more molar refractivity (122.16 esu) than 3A and 3C and correlates well with the calculated binding affinity and experimental antimicrobial data.

A near-infrared absorbing ring-fused quinoid-bisboron pyrrole dye with double BF2 chelation

Ring-fused quinoid-bisboron pyrrole dyes with double BF2 chelation (QBBP) were prepared for the first time. According to X-ray crystallographic analysis, the centrosymmetric structure of QBBP was confirmed. The core of the central quinone moiety and two ring-fused pyrroles is almost planar, and the arrangement of aggregates involve J-type packing. Bisboron dye QBBP underwent two successive one-electron reduction and two successive one-electron oxidation processes. QBBP absorbs in the near-infrared (NIR) with a high molar extinction coefficient. Although QBBP shows no fluorescence and low intersystem crossing (ISC) efficiency, it has considerable photothermal conversion efficiency and is desirable for a photothermal-photodynamic therapy agent.

Micro-Optical Line Generator Metalens for a Visible Wavelength Based on Octagonal Nanopillars Made of Single-Crystalline Silicon

Conventionally, silicon is less often selected as the material of dielectric metasurfaces in the visible band than other lossless materials, including titanium dioxide, silicon nitride, and gallium nitride. The reason is its relatively high extinction coefficient and resulting low transmittance. This study demonstrated that accurately designed nanopillars made of single-crystal silicon could be used satisfactorily on a metasurface, even in the visible band. Four line-focusing metasurface lenses were designed to verify the lens performance effectiveness of silicon nanopillars in the visible band. In addition, a combination of the character projection and the variable-shaped beam modes in the electron beam lithography was operated to evaluate the compatibility between mass productivity and accuracy. We successfully obtained a highly efficient line-focusing metasurface lens composed of single crystalline silicon nanopillars. The parameters of the metasurface lens at a wavelength of 532 nm were as follows: lens thickness, 300 nm; focal length, 3.91 mm; square aperture, 2 mm; numerical aperture, 0.25; measured transmittance, 38.4% to 46.8%; and measured beam spot width, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.68\,\, \mu \text{m}$ </tex-math></inline-formula> (full width at half maximum, FWHM) at the focal point. The results obtained in this study show a promising use of silicon metasurface for optical sensor applications in the visible band.

Synthesis and structural analysis of push-pull imidazole-triazole based fluorescent bifunctional chemosensor for Cu2+ and Fe2+ detection

Fluorescent imidazole-triazole based ligands L1 and L2 have been designed as chemical push-pull chemosensors for divalent metal ions and synthesized through palladium-catalyzed cross-coupling reactions using indium organometallics and click chemistry. The novel ligands exhibit intense absorption in the ultraviolet region with high molar extinction coefficients, and strong fluorescence emission with large Stokes displacements. On the basis of UV–Vis absorption spectroscopy and fluorescence emission data in acetonitrile, L1 is shown as a bifuncional chemosensor with differential response for Fe2+ and Cu2+ over a range of selected 3d divalent and other metal ions. The binding site of the ligand was established by single-crystal X-ray diffraction and 1H NMR spectroscopy studies. The association constants, determined by spectrofluorimetric titrations, show a steady binding affinity of L1 for Cu2+ and Fe2+ in comparison with other previously reported fluorescent bidentate chemosensors, offering the lowest limit of detection (LOD) with Cu2+. DFT calculations provide a rationale properly understanding and interpreting the experimentally observed results. Indeed, a mechanism of the different optical responses of L1 towards 3d divalent metal ions is proposed.

(Invited) Dynamics of Porphyrin-Fullerene Charge Transfer and Recombination Via Ultrafast Spectroscopy

Electron donor-acceptor (D-A) interactions have been the subject of intense scrutiny by the scientific community due to their role in a variety of applications, such as solar energy production, photocatalysis, and stimulus-responsive materials. We chose porphyrins and fullerenes for our fundamental studies of D-A interactions, as they are the prototypical D-A pair by virtue of their well-positioned energy levels, low reorganization energies, and the high extinction coefficients of the porphyrin ring in the visible region. Moreover, they give us the opportunity to not only choose between different bound or endohedral metal ions or clusters, but also to connect them in different ways in order to tune their behavior. We have used time-resolved spectroscopic techniques, including transient absorption, fluorescence upconversion, and time-correlated single-photon counting, to untangle charge separation and recombination dynamics upon donor excitation in the femto- to nanosecond regime to gain additional insight into the relationship between structure and behavior.

Aldehyde‐functionalised distyrylbenzene: Photophysical properties and primary amine sensitivity evaluation in solution and solid state

AbstractThe identification of amines is a highly beneficial process in the industrial sector; food corruption, including fish and shellfish and, more importantly, breathing amines, causes dangerous illnesses. In this study, the optical properties and amine sensing of a π‐conjugated organic compound, 1,4‐bis[4‐formylstyryl]benzene (BFSB), containing two aldehyde functional groups, was investigated. BFSB showed ultraviolet–visible absorption in the range of 318‐329 nm with a high molar extinction coefficient (ε = 2.21*105–2.83*105 M−1 cm−1) in the tested solvent with different polarities. BFSB emits in the visible spectrum (420‐514 nm) with Stokes shifts of ca. 6678‐11 991 cm−1. Density functional theory at the B3LYP/6–31 ++ G level of calculation was employed to evaluate the molar structures and frontier orbitals. The combination of BFSB with poly(methyl methacrylate) and polyvinyl alcohol polymers exhibits different photophysical properties. Interaction of this dye with various primary amines in both solution and solid forms leads to the quick formation of an imine group (the reaction of aldehyde with amine and then dehydration). A hypsochromic shift was observed upon treatment of amine with BFSB.

Exploration of sub-bandgap states in 2D halide perovskite single-crystal photodetector

Greater stability of low-dimensional halide perovskites as opposed to their three-dimensional counterparts, alongside their high extinction coefficient and thus excellent emission properties, have made them popular candidates for optoelectronic applications. Topological edges are found in two-dimensional perovskites that show distinct electronic properties. In this work, using Kelvin Probe Force Microscopy, performed on butylammonium lead bromide (BA2PbBr4) single crystals with optical bandgap of ~413 nm, we elucidate the electronic response of the edges and their potential impact on photodetector devices. We show that the charge-carriers are accumulated at the edges, increasing with the edge height. Wavelength-dependent surface photovoltage (SPV) measurements reveal that multiple sub-bandgap states exist in BA2PbBr4. As the edge height increases, the SPV amplitude at the edges reduces slightly more as compared to the adjacent regions, known as terraces, indicating relatively less reduction in band-bending at the surface due possibly to increased de-population of electrons from sub-bandgap states in the upper bandgap half. The existence of sub-bandgap states is further confirmed by the observation of below-bandgap emission (absorption) peaks characterised by spectral photoluminescence and photothermal deflection spectroscopy measurements. Finally, we fabricated a photodetector using a millimetre size BA2PbBr4 single crystal. Noticeable broadband photodetection response was observed in the sub-bandgap regions under green and red illumination, which is attributed to the existence of sub-bandgap states. Our observations suggest edge-height dependence of charge-carrier behaviour in BA2PbBr4 single crystals, a potential pathway that can be exploited for efficient broadband photodetector fabrication.

NIR-II-absorbing conjugated polymers based on tetra-fused isoindigo ribbons for photothermal conversion and photoacoustic imaging

Conjugated polymers (CPs) that have both a high extinction coefficient (ε) and a high photothermal conversion efficiency (η) in the second near-infrared (NIR-II) region (1,000–1,700 nm) are highly desirable for photoacoustic imaging (PAI) and photothermal therapy (PTT). Here, we report that introducing polycyclic, auxochromic, and coplanar effects together into conjugated backbones affords isoindigo-based CPs with strong absorption and high η in the NIR-II region. Water-dispersed nanoparticles (NPs) of the typical polymer P4TTD-OT, with an ε in mass of 37.1 g −1 cm −1 and an η of 72.9% at 1,064 nm, show superior PAI properties. PAI of 40 mm depth at 1,064 nm is achieved at a lower laser fluence of 7.0 mJ cm −2 with chicken breast muscle as a model tissue, while a signal-to-background ratio of up to 42 dB is realized in the in vivo NIR-II PAI using the 4T1 tumor-bearing mouse model. The NPs can also serve as an effective NIR-II nanoagent for PAI-guided PTT. • A strategy for the synthesis of NIR-II-absorbing CPs is proposed • High η up to 72.9% at 1,064 nm is achieved • Tissue penetration depth of PAI signals up to 40 mm is achieved Jiang et al. report that the combination of polycyclic, auxochromic, and coplanar effects can afford conjugated polymers with high NIR-II molar extinction coefficient and photothermal conversion efficiency.

A Perspective on the Force-Induced Heterolytic Bond Cleavage in Triarylmethane Mechanophores

AbstractTriarylmethane derivatives and their corresponding trityl carbocations are among the oldest chemical species synthesized and studied by chemists. The carbocationic platforms are particularly interesting due to their stability, high extinction coefficient, and tunable absorption of light in the visible spectrum, which can be achieved through structural modifications. These stable cations are traditionally obtained through heterolytic cleavage of judiciously designed, parent triarylmethanes by exposure to acids or UV light (λ &lt; 300 nm), and methods based on electrochemistry or radiolysis. Our group has recently discovered that trityl carbocations can be generated also via mechanical stimulation of solid polymer materials featuring triarylmethane units as covalent crosslinks. In this Synpacts contribution, we expand on our previous finding by discussing some intriguing research questions that we aim to tackle in the immediate future.1 Introduction2 The Development of Our First Triarylmethane Mechanophore3 The Potential Reversibility of Triarylmethane Mechanophores4 A General Molecular Platform for Force-Induced, Scissile, Homolytic and Heterolytic Bond Cleavage?5 Conclusion

Exploring Plant-Based Cooking Oils as a Promising Topical Alternative to Sunscreen

Sunscreens protect the skin from the harmful effects of Ultraviolet radiation in the sunlight. However, most commercial sunscreens pose health and environmental hazards, lose effectivity over time, or are simply unpleasant to use, making it imperative to search for greener, safer, and efficacious alternatives. Plant-based oil extractions are promising given their hydrophobicity, nontoxicity, and availability. This paper aims to determine the absorptive capabilities of five common cooking (vegetable) oils using UVA spectrophotometry and analyze the properties of the oils that relate to their efficacy. Oils high in tocols, such as Rice Bran Oil, had the highest extinction coefficient. The analysis suggests that plant-based oil extractions high in phytonutrients with aromatic rings and pi-systems, such as tocols and carotenoids, present the greatest potential to serve as UV chemical filters.

Diketopyrrolopyrrole and perylene diimine-based large π-molecules constructed via C–H direct arylation

Diketopyrrolopyrrole (DPP) and perylene diimine (PDI) dye moieties with unique structure and strong light absorption have been widely explored as building blocks for organic opto-electronic materials. In this work, we implement a proof-of-concept of welding DPP and PDI together as propellers for the design of novel molecules with star-shaped or three-dimensional (3D) geometries and minimized self-aggregation. Three large conjugated molecules Ph(DPP-PDI)3, Py(DPP-PDI)4 and SF(DPP-PDI)4 with different central cores, i.e., benzene, pyrene and spirofluorene, are designed and facilely synthesized via atom- and step-economic C–H direct arylation (DACH) coupling. The optical, electrochemical properties and photovoltaic performance of target molecules with varied geometries, are systematically investigated by UV–vis absorption, cyclic voltammetry and bulk heterojunction devices, which reveal that 3D molecules have a broader light absorption, much higher extinction coefficients and more favorable energy levels compared with mono DPP-PDI precursor. Our study demonstrates that DACH reaction is an effective and extensible method to prepare branched or 3D π-electronic materials, and provides a guideline for the rational design and synthesis of complex π-molecules in a simple way.

Simulation of Amorphous Silicon Carbide Photonic Crystal Absorption Layer for Solar Cells

In this work, the amorphous silicon carbide (a-SiC) with low cost and high extinction coefficient was used as the light absorption layer of solar cells, and the photonic crystal (PC) structure and defect structure were introduced. By optimizing the scatterer shape, structural parameters and defect types of photonic crystal, the absorption efficiency of the light absorption layer was further improved. The results show that the photonic crystal absorption layer with vacancy line defect is better than the perfect photonic crystal absorption layer. Meanwhile, the absorption efficiency of the photonic crystal absorption layer significantly improves in the case that the scatterer is an elliptical cylindrical air hole scatterer. When the incident light is in the wavelength range of 0.30~0.80 μm and the absorption layer height is 0.60 μm, the absorption efficiency of the absorption layer can reach 95.60%. Compared with the absorption layer without photonic crystal structure, the absorption layer is increased by 43.24%. At the same time, the absorption layer has little dependence on the incidence angle of sunlight. When the incidence angle is 65°, the absorption efficiency is still higher than 80%.

A New Class of Solution Processable Pyrazino[2,3‐g]quinoxaline Carbazole Derivative Based on D–A–D Architecture for Achieving High EQE in Yellow and White OLEDs

AbstractA new pyrazino[2,3‐g]quinoxaline carbazole derivative with branched alkyl peripheral chains PQC‐12b with a donor–acceptor–donor nonplanar architecture is reported. PQC‐12b exhibits high molar extinction coefficients, low bandgap, bright emission behavior in solution and solid‐state, along with positive solvatochromism. PQC‐12b is used in organic light‐emitting diodes (OLEDs) either as sole emissive material or as a guest in different host materials. Amongst these different host–guest OLEDs, CBP‐based device with the optimized concentration of PQC‐12b shows maximum values for power efficacy (PEmax) of 14.1 lm W−1, current efficacy of 26.8 cd A−1, external quantum efficiency (EQEmax) of 8.0%, and lifetime of 5.1 h at an initial luminance of 5000 cd m−2 with a low power efficacy roll‐off and roll‐up character in current efficacy and EQE. A triplet exciton harvesting approach is employed by incorporating thermally activated delayed fluorescent emitter 4CzIPN to obtain a PEmax of 33.4 lm W−1, a CEmax of 56.3 cd A−1, an EQEmax of 15.3%, and a maximum luminance (Lmax) of 29 400 cd m−2. Utility of the compound in solution‐processable white OLED is explored and interestingly showed a PEmax of 42.2 lm W−1, a CEmax of 40.3 cd A−1, an EQEmax of 12.0%, and Lmax of 24 667 cd m−2.

The Meta-Substituted Isomer of TMPyP Enables More Effective Photodynamic Bacterial Inactivation than Para-TMPyP In Vitro.

Porphyrinoid-based photodynamic inactivation (PDI) provides a promising approach to treating multidrug-resistant infections. However, available agents for PDI still have optimization potential with regard to effectiveness, toxicology, chemical stability, and solubility. The currently available photosensitizer TMPyP is provided with a para substitution pattern (para-TMPyP) of the pyridinium groups and has been demonstrated to be effective for PDI of multidrug-resistant bacteria. To further improve its properties, we synthetized a structural variant of TMPyP with an isomeric substitution pattern in a meta configuration (meta-TMPyP), confirmed the correct structure by crystallographic analysis and performed a characterization with NMR-, UV/Vis-, and IR spectroscopy, photostability, and singlet oxygen generation assay. Meta-TMPyP had a hypochromic shift in absorbance (4 nm) with a 55% higher extinction coefficient and slightly improved photostability (+6.9%) compared to para-TMPyP. Despite these superior molecular properties, singlet oxygen generation was increased by only 5.4%. In contrast, PDI, based on meta-TMPyP, reduced the density of extended spectrum β-lactamase-producing and fluoroquinolone-resistant Escherichia coli by several orders of magnitude, whereby a sterilizing effect was observed after 48 min of illumination, while para-TMPyP was less effective (p < 0.01). These findings demonstrate that structural modification with meta substitution increases antibacterial properties of TMPyP in PDI.

Stabilization of Gold Nanoparticles by Hairpin DNA and Implications for Label-Free Colorimetric Biosensors.

With extremely high extinction coefficients and other unique optical properties, gold nanoparticles (AuNPs) have received growing interest in developing biosensors. DNA hairpin structures are very popular probes for the detection of not only complementary DNA or RNA but also aptamer targets. This work aims to understand the effect of the structure and sequence of hairpin DNA for the stabilization of AuNPs and its implications in AuNP-based label-free colorimetric biosensors. A series of hairpin DNA with various loop sizes from 4 to 26 bases and sequences (random sequences, poly-A and poly-T) were tested, but they showed similar abilities to protect AuNPs from aggregation. Using hairpin DNA with a tail under the same conditions, optimal protection was achieved with a six-base or longer tail. DNA hairpins are likely adsorbed via their tail regions or with their terminal bases if no tail is present. Molecular dynamics simulations showed that the rigidity of the hairpin loop region disfavored its adsorption to AuNPs, while the flexible tail region is favored. Finally, a DNA sensing assay was conducted using different structured DNA, where hairpin DNA with a tail doubled the sensitivity compared to the tail-free hairpin.