Triangular Gold Nanoplates Research Articles

Early Detection of Carcinoembryonic Antigen Using Hollow Nanospheres Improves the Outcome of Pediatric Solid Tumors

A sandwich electrochemical immunosensor was developed for sensitive detection of carcinoembryonic antigen (CEA) using hollow nanospheres composite material loaded with gold nanoparticles as the signal tag and gold triangular nanoplate as the substrate material. Samples obtained from pediatric solid tumor tissue and their blood samples were used to detect CEA using this nanoparticle. The hollow nanospheres constructed in this study, provided abundant reaction sites and high catalytic activity, while the loading of Au NPs enhances conductivity and biocompatibility. The immunosensor has good analytical ability in detecting CEA in the mass concentration range of 1 pg/mL to 50 ng/mL, with a low detection limit (LOD) of 0.37 pg/mL. This novel method provides a valuable tool for rapid clinical detection of CEA in the context of pediatric solid tumors.

Colorimetric and visual sensing of ferrous ion by Fenton reaction-stimulated etching of triangular gold nanoplates

In this study, a method for ultrasensitive sensing of Fe2+ based on Fenton reaction mediated etching of triangular gold nanoplates (Au NPLs) was developed. In this assay, the etching of Au NPLs by H2O2 was accelerated in the presence of Fe2+ due to the generation of superoxide free radical (O2·-) via Fenton reaction. With the concentration of Fe2+ increased, the shape of Au NPLs changed from triangular to sphere with the blue shifted localized surface plasmon resonance, accompanying a series of consecutive color changes from blue, bluish purple, purple, reddish purple and finally to pink. The rich color variations enable rapid visual quantitative determination of Fe2+ within 10 min. A good linear relationship between the peak shifts and the concentration of Fe2+ was obtained in the range of 0.035 to 1.5 μM (R2 = 0.996). Favorable sensitivity and selectivity in the presence of other tested metal ions were achieved in the proposed colorimetric assay. The detection limits (3ơ/k) for Fe2+ was 26 nM by UV–vis spectroscopy, and the clearly discernible concentration of Fe2+ was as low as 0.07 μM by naked eyes. The recoveries of fortified samples in pond water and serum samples ranged from 96% to 106% with interday relative standard deviations <3.6% in all cases, demonstrating the applicability of the assay for measuring Fe2+ in real samples.

Plasmonic nanoprobes on single AuNTs for evaluating and monitoring the dynamic release of 2D drug carriers.

The use of nanomaterials in drug delivery has gained significant attention in recent years. In this project, we developed a novel localized surface plasmon resonance (LSPR) nanoprobe on single gold triangular nanoplates (AuNTs) for dynamic monitoring of the drug carrier release process. Graphene, as the drug carrier, could be immobilized on the AuNT surface through the π-π* stacking effect. Upon loading or releasing the model drug (doxorubicin, DOX), subtle changes in the local microenvironment's dielectric constant around the AuNTs induced notable red-shifts or blue-shifts in the LSPR scattering spectra of single AuNTs. Furthermore, the spectral shifts led to a continuous enhancement in the red channel of the dark field microscopy (DFM) images during the drug release process in vitro, demonstrating that the drug release system is not susceptible to potential confounding factors. These release kinetics results under different conditions could be well-fitted using the Higuchi desorption model, further proving that this nanoprobe could be employed for evaluating the controlled release ability of 2D nanocarriers. These findings are expected to inspire new ideas and technologies in the preparation of more effective drug carriers, making a significant contribution to the development of drug delivery nanosystems and nanomedicine.

Triangular gold nanoplates integrated microgel-based sensor for urinary tract infection and glucosuria detection

A glucose-responsive photonic crystal integrated optical sensor that can detect glucosuria and urinary tract infection was designed by comprehensive numerical analysis. We showed a comparative study of nanoparticle shapes and materials and discussed the performance parameters of the sensor, such as the signal-to-noise ratio, the sensitivity, and the detection limit for various pH levels. Normally distributed nanoparticles were embedded within a functionalized microgel matrix in this proposed structure, and the theoretical study was conducted using the finite element method. Gold was selected as a photonic crystal material due to its nontoxicity and better sensing performance, and triangular-shaped nanoplates were chosen due to their better surface plasmon resonance performance. The sensor showed improved sensitivity of &lt;85.65 nm/mM for pH 7.4, and &lt;110.60 nm/mM for pH 8.0. It depicted a lower detection limit of &gt;0.34 mM for pH 7.4 and &gt;0.30 mM for pH 8.0 than those of previously reported glucose sensor studies. Moreover, the proposed glucose sensor exhibited a practicable signal-to-noise ratio and sensor resolution range. This label-free sensor manifested additional features such as reusability of the sensor, high sensing reversibility, and being readily usable at point-of-care medical facilities.

Gold triangular nanoplates with edge effect for reaction monitoring under dark-field microscopy

True morphology imaging of individual nanoprobes with high sensitivity and in situ dynamic response is crucial in the dark-field microscopy imaging analysis. However, the geometry and orientation of the probes are experimentally challenging to visualize as their physical dimensions are usually below the diffraction limit. In this work, two-dimensional triangular gold nanoplates (AuNPLs) with the edge length > 600 nm and the thickness ~25 nm displayed clear triangular-shaped far-field scattering patterns (TFSPs) under a dark-field imaging system owing to the strong electrical field strength around the sharp edges and tips. Thus, their actual shape and orientation specific scattering image are directly visualized only by dark-field microscopy and furtherly applied to real-time monitor the etching and alloying progress of AuNPLs. The results showed that the reactive sites of AuNPLs are located at their vertices and sharp edges owing to the sensitive edge effect. This work provides a new perspective to directly monitor the conformation and dynamics of chemical reaction at the single particle level.

Folic Acid and Chitosan-Functionalized Gold Nanorods and Triangular Silver Nanoplates for the Delivery of Anticancer Agents.

BackgroundAdvances in the field of nanotechnology have shed light on the applications of nanoparticles for cancer treatment.MethodsFolic acid and chitosan-functionalized gold nanorods (FACS-R) and triangular silver nanoplates (FACS-T) were synthesized and their properties were elucidated by UV-visible spectrophotometry, Fourier-transform infrared spectroscopy, field emission transmission electron microscopy and high-resolution X-ray diffraction.ResultsThe average size of the FACS-R was determined to be a transverse length of 13.1 ± 1.8 nm and a longitudinal length of 47.2 ± 8.9 nm with an aspect ratio of 3.6. The average size of FACS-T was measured to be 31.8 ± 7.7 nm. Colloidal solutions of FACS-R and FACS-T were stable on the shelf at ambient temperature for 14 days in the dark. Anticancer agents were encapsulated in FACS-R and FACS-T. FACS-T showed a higher encapsulation efficiency with docetaxel, paclitaxel and diallyl disulfide than FACS-R. The cell viability on human gastric adenocarcinoma cells (AGS), human epithelial cervix adenocarcinoma cells (HeLa) and human colorectal adenocarcinoma cells (HT-29) after treatment with anticancer agent-encapsulated FACS-R and FACS-T was determined using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Interestingly, paclitaxel-encapsulated FACS-R and FACS-T showed the highest percentages of early and late apoptosis on HeLa cells. A cell cycle analysis demonstrated increased G2/M arrest on HeLa cells with docetaxel and paclitaxel-encapsulated FACS-R and FACS-T. The FACS-T induced more G2/M arrest on HeLa cells than the FACS-R. To assess applications in near-infrared photothermal therapy (PTT), the cell viability on HeLa cells with the anticancer agent-encapsulated FACS-R and FACS-T was assessed in the presence or absence of 808 nm laser irradiation. The results showed that 808 nm laser irradiation significantly decreased cell viability.ConclusionCollectively, the triangular silver nanoplates were more effective than the gold nanorods for PTT. We believe that as-prepared nanoparticles have remarkable features and will become promising future nanomedicine.

Triangular gold nanoplates/two-dimensional nano mica platelets with a 3D lightning-rod effect as flexible nanohybrid substrates for SERS bacterial detection.

Triangular gold nanoplates (TAuNPs) were prepared by a one-step rapid growth method and then reduced and stabilized on two-dimensional nano mica nanoplatelets (NMPs). We also prepared TAuNP/NMP nanohybrids with a three-dimensional lightning-rod effect by oxidative etching. The surface of the delaminated NMPs (only 1 nm thick) is highly charged and can provide a large specific surface area; thus, it can be used as a substrate for the stable growth of gold nanoplates. In addition, by controlling relevant synthesis parameters, the edge length of the TAuNPs can be easily adjusted in the range of 30-90 nm. During reduction of the TAuNPs, the cationic surfactant cetyltrimethylammonium chloride was added as a protective agent to surround the TAuNPs; consequently, the surface was positively charged, which facilitates adsorption for detecting molecules with negative charges. When nanohybrids were used in surface-enhanced Raman spectroscopy (SERS) to detect adenine molecules, the limit of detection concentration was 10-9 M. The Raman enhancement factor was 5.7 × 107, and the relative standard deviation (RSD) was 9.8%. Finally, this method was applied to the biological detection of Staphylococcus aureus, and the surface charge and hydrophilic properties of the material significantly improved the SERS signal of S. aureus. The limit of detection concentration was 102 CFU mL-1, and the RSD was 11.2%. The TAuNP/NMP nanohybrids can provide very rapid and sensitive SERS detection of biomolecules.

Triangular gold nanoplates as saturable absorber for passively Q-switched fiber laser at 1.56 μm

Wideband pulsed fiber lasers have been realized by using gold nanomaterials as saturable absorber (SAs). Compared to other morphologies, triangle gold nanoplates (TGNs) have multiple localized surface plasmon resonance peaks, which means that it can be used for construct wideband pulse laser. In this work, we propose and demonstrate an all-fiber passively Q-switched fiber laser at 1.56 μm by using TGNs as SA. The modulation depth and saturation intensity of the TGNs SA measured at 1560 nm are 7.1% and 13.8 MW cm−2, respectively. By putting the TGN SA into the laser cavity pumped by a 980 nm laser diode, stable passively Q-switched laser at 1568 nm was achieved for a threshold pump power of 40 mW. The pulse duration decreases from 11.4 μs to 4.2 μs and the repetition rate increases from 12.5 kHz to 20 kHz with the increasing of the pump power from 40 mW to 75 mW. Our results indicate that the TGNs is a type of versatile broadband SA material for construct pulse lasers.

Enhancement of Optical Nonlinearity in the Triangular Gold Nanoplates on Indium Tin Oxide

The nonlinear optical materials with small footprint, low operating energy, ultrafast and large nonlinear optical response are highly required for the signal processing. Here, we investigated the broadband optical nonlinear response of the triangular gold nanoplates on indium tin oxide (ITO) coated glass, and obtained a large optical nonlinear enhancement at its localized surface plasmon resonance (LSPR) region at the wavelength of 780 nm, far from the ENZ (epsilon-near-zero) region of ITO. The measured nonlinear refractive index and nonlinear absorption coefficient can reach up to 8.39×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /W and 5.81×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-8</sup> m/W, respectively. The experimental results highlight the noble metals' potential for nonlinear optics and nanophotonics.

Online Monitoring Strategies for Colorimetric Detection of Cadmium Ions and pH Based on Gold Nanomaterials with a Low-Cost Color Sensor

Gold triangular nanoplates (AuTNPs) and gold nanorods (AuNRs) have been widely used in colorimetric sensing due to their shape-dependent localized surface plasmon resonance properties. Herein, we report new colorimetric detection strategies based on a low-cost RGB colorimetry system equipped with an industrial-grade color sensor and provide two demonstrations for the potential applications. The first is the online detection of cadmium ions (Cd²⁺) through the Cd²⁺-promoted etching of glutathione-stabilized AuTNPs that was initialized by H₂O₂ and iodide. For the first time, the etching time corresponding to the inflection point of the first derivative of the Red channel transmission curve was used for calibration. The method was finally applied in the detection of Cd²⁺ in fortified rice samples. In the second demonstration, the RGB values continuously acquired using the instrument were converted to HSV (hue, saturation, and value) space, which was successfully employed in pH detection via etching of AuNRs. Importantly, the overaged AuNRs with poor homogeneity could be discerned by the method at the beginning of analysis, which is helpful in achieving reliable measurements. Our results suggest that integration of the low-cost portable instrument and the new strategies has the promising potential for sensitive, accurate, precise, and good-linearity detection.

A novel colorimetric sensor array for Baijiu discrimination based on gold triangular nanoplates etching

Abstract The detection of Baijiu is important because of its essential role in Chinese food culture. In this study, a novel acid- and sulfide-sensitive colorimetric sensor array was constructed for Baijiu discrimination based on the etching of gold triangular nanoplates (Au TNPs). Under acidic conditions, Au TNPs were easily etched by potassium iodide (KI) and hydrogen peroxide (H2O2). The morphology of Au TNPs underwent transitions from triangle to ellipse or round, accompanied by multiple color changes. Acids or sulfides existed in Baijiu could further accelerate or inhibit Au TNPs etching, forming a unique colorimetric response pattern, which would be processed by pattern recognition. With this method, 6 kinds of acids and 7 kinds of sulfides were accurately identified, respectively. Quantitative analysis of glutathione had also been achieved. The Euclidean distance (ED) showed a linear relationship with the logarithm of glutathione concentration in the range of 0.1−10 mM. 16 kinds of Baijiu were successfully discriminated with an accuracy of 100 %. Notably, several interfering alcoholic beverages were also accurately identified. The colorimetric sensor array constructed in this work exhibited the advantages of simplicity of operation, quick response and superior performance, showing potential application prospects either for the real-time or on-site identification of traditional Chinese Baijiu.

Visual detection of different metal ions based on the tug of war between triangular Au nanoparticles and metal ions against mercaptans.

Herein, a new colorimetric sensor array was developed for the first time, which can rapidly recognize 9 types of metal ions (e.g., Ni2+, Zn2+, Cd2+, Cu2+, Cr3+, Fe2+, Se2+, Mn2+, and Mg2+). The colorimetric characteristics of the sensor array were closely related to the oxidation etching of the triangular gold nanoplates (AuNPLs) by hydrogen peroxide (H2O2), catalyzed by horseradish peroxidase (HRP). In the design, two types of thiols (glutathione (GSH) and cysteine (Cys)) as recognition elements were employed to construct the sensor units (AuNPLs/GSH and AuNPLs/Cys) and adjust the etching degrees of AuNPLs in the presence of various metal ions. The differential binding affinities between metal ions and thiols will lead to different degrees of oxidation etching of AuNPLs with hydrogen peroxide, exhibiting characteristic colors, which can be visually distinguished by the naked eye. Thus, the colorimetric sensor array provides a new way for the discrimination of various metal ions, thereby simplifying the water quality analysis.

Multifunctional shape-dependent plasmonic nanoprobe by enzymatic etching of single gold triangular nanoplate

Hydrogen peroxide (H2O2), as a signaling molecule, plays a vital role in a wide variety of signaling transduction processes, aging, and diseases. However, the excessive production of H2O2 causes various diseases. Herein, we develop a novel method for H2O2 detection in live cells via dark-field scattering spectroscopy with gold triangular nanoprisms (AuTNPs) as probes. The corners of AuTNPs would be gradually oxidatively etched by the strong coordination of Br• which is generated by enzymatic reactions in the presence of horseradish peroxidase (HRP), bromide ion and trace hydrogen peroxide. Benefitting from the morphological change, the single AuTNP based plasmonic nanoprobe shows notable blueshifts and scattering color changes which could be real-time monitored under the dark-field microscopy. The peak position in the scattering spectra of individual AuTNP blueshifts linearly with the increase of H2O2 concentration, and exhibits high sensitivity to H2O2 in a large range from 2.5 to 100 µM with a low detection limit (LOD) of 0.74 µM. Moreover, the experimental results were supported by the simulated results via the finite-difference time-domain (FDTD) method. The nanoprobes have been further used for intracellular H2O2 detection in live cells. Besides, the etching of AuTNP also provides an alternative method to design novel plasmonic logic chips and write-once plasmonic memories.

A multicolor colorimetric assay for sensitive detection of sulfide ions based on anti-etching of triangular gold nanoplates

This study presents a viable colorimetric assay for sensitive sensing of sulfide ions (S2−) using triangular gold nanoplates (AuNPLs) as nanoprobes. The sensing mechanism is based on regulating the etching process of AuNPLs upon the addition of different concentration of S2−. In the presence of Cu2+ and I−, the AuNPLs would be etched by triiodide ions (I3−) produced from the redox reaction between Cu2+ and I−, resulting in the transformation of the shape of AuNPLs from triangular into sphere and blue shifted localized surface plasmon resonance (LSPR). However, in the presence of S2−, a preferential reaction between Cu2+ and S2− occurred, and the inhibition of the formation of I3− prevented the AuNPLs from being etched. Correspondingly, a red shift of LSPR can be observed with an increase of the concentration of S2−, accompanying a readily distinguishable color transition that goes from pink to purplish red, purple, bluish purple and blue. The color alterations allow for a visual estimation of the concentrations of S2−, and the lowest visual concentration through naked-eyes was 0.1 μM. A good linear relationship (R2 = 0.997) between the peak shifts and the concentration of S2− was obtained in the range of 0.02–1.5 μM, and the limit of detection (3 s/k) by UV–vis spectroscopy was as low as 16 nM. The as-established assay displayed excellent selectivity, which was successfully applied for analysis of S2− in environmental water samples.

Sequential colorimetric sensing of cupric and mercuric ions by regulating the etching process of triangular gold nanoplates.

Atriangular gold nanoplate (AuNPL)-based colorimetric assay is presentedfor ultrasensitive determination of cupric ions (Cu2+) and mercuric ions (Hg2+) in sequence. AuNPLs were found to be etched efficiently when producing triiodide ions (I3-) by a redox reaction between Cu2+ and iodide ions (I-), leading to a change ofthe shape of AuNPLs from triangular to sphere along with a color change from blue to pink. Inthe presence of Hg2+ the etching of AuNPLs was suppressed due to the consumption of I- by the formation of HgI2. With an increase of the concentration of the Hg2+ a transformation from sphere to triangular in the shape of AuNPLs occurredwith a color change from pink to blue. The evolution of AuNPLs from etching to anti-etching state by sequential addition of Cu2+ and Hg2+ was accompanied with color variations and band shifts of localized surface plasmon resonance (LSPR), allowing for visual and spectroscopicdetermination of Cu2+ and Hg2+ successively within 15min. In the range 0.01-1.5μM for Cu2+ and 0.02-3.0μM for Hg2+, the linear relationship between the band shift values and the target ions concentration was found good (R2 > 0.996). The limit of detections (3S/k) was 19nM for Cu2+ and 9nM for Hg2+, respectively. The lowest visual estimation concentration was 80nM for both Cu2+ and Hg2+ through the distinguishable color changes. This system exhibited desirable selectivity for Cu2+ and Hg2+ over other common ions tested. The method has been successfully applied tosequential determinationof Cu2+ and Hg2+ in real water and food samples. Graphical abstract Scheme 1 Schematic illustration for sequential detection of Cu2+ and Hg2+ based on etching of AuNPLs.

(Gold triangular nanoplate core)@(silver shell) nanostructures as highly sensitive and selective plasmonic nanoprobes for hydrogen sulfide detection.

Hydrogen sulfide plays a significant role in living beings, while its abnormal concentration is related to many diseases. Besides, H2S gas is harmful to human beings and the environment. The detection of H2S has therefore attracted much attention in the past several decades. Herein, highly sensitive and selective H2S plasmonic nanoprobes (gold triangular nanoplate core)@(silver shell) (AuTNP@Ag) are reported. By virtue of the high refractive index sensitivity of Au TNPs to the surrounding medium and facile sulfurization of silver by sulfur ions, AuTNP@Ag exhibits great sensitivity to both sulfur ions and H2S gas. The shifts of the plasmon peak are as large as 16 nm for the ventilation of 1 ppm hydrogen sulfide. AuTNP@Ag nanoprobes also exhibit very good sensing linearity at low concentrations of sulfur ions. Moreover, excellent sensing selectivity for sulfur ions is obtained. A type of test gel, which can produce a naked-eye observable color change when exposed to 1-100 ppm hydrogen sulfide gas, is developed using AuTNP@Ag nanoprobes. Owing to the high sensitivity, linearity, and selectivity of the Au TNP@Ag nanoprobes for hydrogen sulfide sensing, this work paves the way for the plasmonic detection of hydrogen sulfide in both biological and environmental applications.

Synthesis of fluorescent, triangular gold nanoplates through surface capping by a cationic diacetylene

In this work triangular gold nanoplates (AuNTPs) are prepared by spontaneous conversion of spherical-like gold precursors induced by the shape-directing properties of the diacetylene N,N,N-trimethylpentacosa-10,12-diyn-1-ammonium bromide (PCD_ABr), employed as capping agent. In detail, thin AuNTPs with edge length below 200 nm are obtained by incubating premade isotropic gold nanoparticles protected with chitosan (Chit@AuNPs) in PCD_ABr solutions through a standard ligand-exchange reaction (LER) carried out in water at room temperature. Control of the AuNTPs size as well as the reaction shape-yield up to 65% is achieved by simply varying the reagents ratio in the incubation mixture.Because of the high anisotropy of the flat nanoparticles, the AuNTPs-containing hydrosols show intense, well-defined absorptions in the NIR spectral region, which make them excellent candidates as biosensors also for in vivo assays with biological tissues and fluids.When subjected to UV light, both triangular and nearly spherical PCD_ABr-coated gold nanoparticles exhibit fluorescent properties and Raman features typical of disordered, sp2-based carbon nanostructures. The photogeneration of graphite-like moieties within the diacetylene shell instead of expected polydiacetylene skeleton is most likely due to poor alignment of the monomer chains on the gold surfaces and seems to be favored on isotropic nanoparticles.

Gold Triangular Nanoplates Based Single-Particle Dark-Field Microscopy Assay of Pyrophosphate.

Owing to the sharp edge-enhanced electric field, gold triangular nanoplates (AuNPLs) exhibit strong localized surface plasmon resonance (LSPR) scattering and display as red spots with high intensity and narrow distribution under dark-field microscopy (DFM), based on which sensitive sensing applications could be developed. As a concept of proof, the inhibition effects of pyrophosphate (PPi) against the etching of AuNPLs based on Cu2+ and I- mediated is dynamically monitored. The etching proceeding of AuNPLs by copper ions and iodide ions leads to the gradually blue-shifted LSPR scattering color of AuNPLs under DFM from the original red to green, accompanied by the reduced LSPR scattering intensity. As a result of the strong affinity of PPi to copper ions, the presence of PPi makes the etching process greatly suppressed, thereby achieving sensitive detection of the PPi.

Using a Graphene-Polyelectrolyte Complex Reducing Agent To Promote Cracking in Single-Crystalline Gold Nanoplates.

It is a challenge to produce single-crystalline gold nanoparticles having regular size definition designed for controlled light absorbance and internal structural inhomogeneities to enhance electro-magnetic fields. Here, we report a synthetic strategy to generate large single-crystalline triangular or hexagonal gold nanoplates with multiple cracks within the plates using a graphene-polyelectrolyte complex as both a surface adsorbent and bulk reducing agent. Large-scale gold nanoplates can be synthesized within 48 h. First-principles calculations indicate that the nanoplates have a kinetically limited morphology resulting from prior growth of {111} facets confined by the graphene-polyelectrolyte multilayer. The nanocracks result from the inability of the bulk reducing agent to enter narrow defect spaces during growth that remained permanently. The nanoplates had extraordinary physical-chemical detection sensitivity when used for surface-enhanced Raman scattering (SERS) and surface-enhanced infrared absorption (SEIRA). The limit of rhodamine 6G (Rh6G) SERS detection is as low as 5 × 10-13 M. The gold nanoplates also showed a remarkable light-to-heat conversion efficiency (68.5%). The approach described may be applicable to other metals so that tunable nanostructures can be generated by the graphene-polyelectrolyte multilayer strategy.

A gut check

Biomaterials A large proportion of the population suffers from gastrointestinal disease involving either chronic conditions like Crohn's disease or acute conditions like peptic ulcers. It would be ideal to be able to track changes in local conditions, like pH, with functional changes such as motility. Huang et al. developed an ingestible sensor based on gold triangular nanoplates coated with polyaniline (PANI), as both materials are stable across a wide pH range. The gold shows strong near-infrared absorption around 1200 nanometers, whereas PANI absorbs around 790 nanometers, but only PANI shows absorption changes with changing pH. Photoacoustic imaging is used to detect changes in absorption, and thus pH, while also allowing for real-time tracking of gastric contractions, gastric emptying, and transit time. ACS Nano 10.1021/acsnano.9b04541 (2019).