The authors describe a colorimetric method for the determination of the staphylococcal enterotoxin B (SEB) that also allows for visual readout. The assay is based on the growth of gold nanoparticles (AuNPs) mediated by a hemin/G-quadruplex DNAzyme which generates a color change from red to blue in the presence of SEB. The method is enzyme-free and does not require a label. The kinetics of the formation of the AuNPs is controlled by the hemin/G-quadruplex DNAzyme and this is key to the signal generation mechanism. In the presence of SEB, the reactions between aptamer and target modulated the amount of single probe G strands that form DNAzyme capable of consuming hydrogen peroxide. The growth process of AuNPs is influenced by the resulting concentration of H2O2 and leads to the color change. Under optimal conditions, a linear relationship exists between absorbance and SEB concentration in the range from 0.1 to 500 pg·mL￣1 which covers the clinically relevant range. In case of visual detection, the lower limit of detection is 1 pg·mL?1. The assay described here is sensitive, comparably inexpensive and can detect SEB rapidly without the need for sophisticated equipment. In our perception, the method has a wide scope in that it may be adapted to various nucleic acids, proteins and other biomolecules if respective aptamers are available.
An isothermal colorimetric method is described for amplified detection of the CaMV 35S promoter sequence in genetically modified organism (GMO). It is based on (a) target DNA-triggered unlabeled molecular beacon (UMB) termini binding, and (b) exonuclease III (Exo III)-assisted target recycling, and (c) hemin/G-quadruplex (DNAzyme) based signal amplification. The specific binding of target to the G-quadruplex sequence-locked UMB triggers the digestion of Exo III. This, in turn, releases an active G-quadruplex segment and target DNA for successive hybridization and cleavage. The Exo III impellent recycling of targets produces numerous G-quadruplex sequences. These further associate with hemin to form DNAzymes and hence will catalyze H2O2-mediated oxidation of the chromogenic enzyme substrate ABTS2? causing the formation of a green colored product. This finding enables a sensitive colorimetric determination of GMO DNA (at an analytical wavelength of 420 nm) at concentrations as low as 0.23 nM. By taking advantage of isothermal incubation, this method does not require sophisticated equipment or complicated syntheses. Analyses can be performed within 90 min. The method also discriminates single base mismatches. In our perception, it has a wide scope in that it may be applied to the detection of many other GMOs.
Graphical abstract An isothermal and sensitive colorimetric method is described for amplified detection of CaMV 35S promoter sequence in genetically modified organism (GMO). It is based on target DNA-triggered molecular beacon (UMB) termini-binding and exonuclease III assisted target recycling, and on hemin/G-quadruplex (DNAzyme) signal amplification.
The article describes a colorimetric assay for the determination of thrombin. It is based on the application of a triple enzyme-mimetic activity and a dual aptamer binding strategy. The triple signal amplification relies on oxidation of the chromogenic enzyme substrate 3,3,5,5-tetramethylbenzidine (TMB) that is catalyzed by composites consisting of graphene oxide (GO), gold/platinum nanoparticles (AuPtNP), and aptamer (Apt15), a G-quadruplex/hemin conjugate. The dual-aptamer target binding strategy is based on the fact that thrombin has two active sites to be recognized by its aptamers (Apt15 and Apt29). Magnetic beads (MBs) were modified with Apt29 (Apt29-MB) and then are bound by the GO-AuPtNP-Apt15/G-quadruplex/hemin composites. In the presence of thrombin, Apt29-MB and the GO-AuPtNP-Apt15/G-quadruplex/hemin composites form a sandwich-like superstructure. Thus, the absorbance increases due to the formation of TMB oxide produced by catalysis of the composites. Under optimized conditions, the absorbance at 450 nm increases linearly in the 0.30 to 100 nM thrombin concentration range, and the limit of detection is 0.15 nM. The method is simple, rapid, and does not require complicated instrumentation. Bovine serum albumin, human serum albumin and other proteins were found not to interfere.
Graphical abstract Schematic presentation of the photometric thrombin assay based on a triple enzyme-mimetic activity of combined nanomaterials (consisting of GO, AuPtNPs and the G-quadruplex/hemin DNAzyme) and two aptamers TMB: 3,3,5,5-tetramethylbenzidine, TMBox: 3,3,5,5-tetramethylbenzidine oxide, AuPtNP: gold/platinum nanoparticles).
A toehold-aided DNA recycling amplification technology was developed based on the combination of toehold-aided DNA recycling and the hemin/G-quadruplex label. The dsDNA formed between aptamer and DNA1 was first immobilized on magnetic beads. On addition of target analyte (exemplified here for riboflavin), the aptamer-riboflavin complex is formed and DNA1 is released by the beads. After magnetic separation, the supernatant containing the released DNA1 is added to a solution containing the hairpin capture DNA on magnetic beads. DNA1 will hybridize with the hairpin capture DNA via toehold binding and branch migration. This process will open the hairpin structure, and an external toehold is formed in the newly formed dsDNA. On addition of reporter DNA containing the G-quadruplex, it will interact with the formed dsDNA via toehold binding and branch migration, resulting in the releasing of DNA1 and capturing of reporter DNA on the magnetic beads. The released DNA1 will bind to another hairpin capture DNA which can start another round of DNA1 recycling. Chemiluminescence (CL) is generated by the G-quadruplex-hemin-luminol CL reaction system. Under optimal conditions, the calibration plot is linear in the 0.1 to 700 nM riboflavin concentration range, with a 30 pM detection limit (at a signal-to-noise ratio of 3). The method was successfully applied to the quantitation of riboflavin in spiked urine samples.
An efficient approach is demonstrated for preparing particles consisting of a silver core and a shell of molecularly imprinted polymer (Ag@MIP). The MIP is prepared by using bisphenol A (BPA) as the template and 4-vinylpyridine as the functional monomer. The Ag@MIP fulfills a dual function in that the silver core acts as a SERS substrate, while the MIP allows for selective recognition of BPA. The Ag@MIP is characterized by scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, thermogravimetric analysis and Raman spectroscopy. The Raman intensity of Ag@MIP is higher than that of bare silver microspheres. The detection limit for BPA is as low as 10?9 mol·L?1.
Graphical abstract Schematic illustration of the preparation of silver microspheres coated with a molecularly imprinted polymer (Ag@MIPs) for detecting bisphenol A (BPA) by surface enhanced Raman scattering (SERS).
We report on a new amplification strategy for use in an immunoassay for influenza virus subtype H7N9. Graphene sheets were first placed on a glassy carbon electrode (GCE), and gold nanoparticles were then electrodeposited as a support for a layer of alcohol dehydrogenase (ADH) in a sol–gel containing thiol groups. Protein A was used to properly orientate immobilized antibody against H7N9 on the sol–gel, and this is shown to result in strongly improved specificity of the antigen-antibody binding. Thus, a sensitive and specific immunosensor was obtained in which a quadruple signal amplification strategy is employed, viz. (a) via the use of graphene sheets, (b) via a hybridization chain reaction, (c) the use of hemin/G-quadruplex DNAzyme concatamers, and (d) the use of ADH. The hemin/G-quadruplex is a typical DNAzyme, which simultaneously acts as NADH oxidase and HRP-mimicking DNAzyme. The hybridization chain reaction-based DNAzyme concatamers assembled on multi-walled carbon nanotubes (MWCNTs) and the ADH represent a triple electrocatalytic enzyme cascade system. Sandwich immunoreactions occurred between the capture antibody on the electrode and the secondary antibody labeled with MWCNTs. Positively charged Methylene Blue (MB) was then used as an intercalator to detect the DNAzyme concatamer formed. The differential pulse voltammetric signals for MB are related to the concentration of H7N9 in the range from 8 to 60 pg · mL−1, and the detection limit is 0.81 pg · mL−1 (at an S/N ratio of 3). This immunoassay is very sensitive, specific and robust.
An electrochemical sandwich immunosensor has been developed for sensitive and specific detection of influenza virus subtype H7N9. Protein A was used to properly orientate antibody. The hybridization chain reaction based DNAzyme concatamers assembled on multi-walled carbon nanotubes (MWCNTs) and the ADH represent a triple electrocatalytic enzyme cascade system.
Gas sensor arrays often lack discrimination power to different analytes and robustness to interferants, limiting their success outside of research laboratories. This is primarily due to the widely sensitive (thus weakly-selective) nature of the constituent sensors. Here, the effect of orthogonality on array accuracy and precision by selective sensor design is investigated. Therefore, arrays of (2–5) selective and non-selective sensors are formed by systematically altering array size and composition. Their performance is evaluated with 60 random combinations of ammonia, acetone and ethanol at ppb to low ppm concentrations. Best analyte predictions with high coefficients of determination (R2) of 0.96 for ammonia, 0.99 for acetone and 0.88 for ethanol are obtained with an array featuring high degree of orthogonality. This is achieved by using distinctly selective sensors (Si:MoO3 for ammonia and Si:WO3 for acetone together with Si:SnO2) that improve discrimination power and stability of the regression coefficients. On the other hand, arrays with collinear sensors (Pd:SnO2, Pt:SnO2 and Si:SnO2) hardly improve gas predictions having R2 of 0.01, 0.86 and 0.28 for ammonia, acetone and ethanol, respectively. Sometimes they even exhibited lower coefficient of determination than single sensors as a Si:MoO3 sensor alone predicts ammonia better with a R2 of 0.68.
Graphical abstract Conventional arrays (red) with weakly-selective sensors span a significantly smaller volume in the analyte space than arrays containing distinctly-selective sensors (orthogonal array, green). Orthogonal arrays feature better accuracy and precision than conventional arrays in mixtures of ammonia, acetone and ethanol.
This work reports on a colorimetric platform for determination of chromium ions (Cr3+) and mercury ions (Hg2+) using silver nanoparticles (AgNPs) capped with cytosine triphosphate (CTP). The capped AgNPs were synthesized one-step by reduction of AgNO3 in the presence of CTP. It was found that such AgNPs aggregate in the presence of Cr3+. This results in a decrease in the intensity of the surface plasmon resonance (SPR) band at 390 nm and the formation of a new red-shifted band at 510 nm, and consequently a color change from yellow to red. Different from the Cr3+-induced aggregation of AgNPs, exposure to Hg2+ causes the formation of a mercury layer around the surface of the AgNPs. This, in turn, causes the SPR absorption of the AgNPs to decrease and to undergo a slight blue shift, and this results in a fading of the yellow color. The findings are the basis of developing a new method for quantification of either Cr3+ or Hg2+, with detection limits of 6.25 μM for Cr3+ and of 0.125 μM for Hg2+, respectively. The method was applied to the determination of the two ions in spiked drinking water and lake water samples, and recoveries ranged from 94.5% to 101.3% for Cr3+, and from 96% to 108% for Hg2+, which is satisfactory for quantitative assays performed in water samples.
Graphical abstract Cytosine triphosphate-capped silver nanoparticles (cAgNPs) are shown to represent a viable probe for visual and colorimetric detection of Hg2+ and Cr3+ via two different mechanisms: aggregation of cAgNPs in case of Cr3+; and amalgamation of cAgNPs in case of Hg2+.
An aptamer based assay is described for the colorimetric detection of adenosine. The presence of adenosine triggers the deformation of hairpin DNA oligonucleotide (HP1) containing adenosine aptamer and then hybridizes another unlabeled hairpin DNA oligonucleotide (HP2). This leads to the formation of a double strand with a blunt 3′ terminal. After exonuclease III (Exo III)-assisted degradation, the guanine-rich strand (GRS) is released from HP2. Hence, the adenosine-HP1 complex is released to the solution where it can hybridize another HP2 and initiate many cycles of the digestion reaction with the assistance of Exo III. This leads to the generation of a large number of GRS strands after multiple cycles. The GRS stabilize the red AuNPs against aggregation in the presence of potassium ions. If, however, GRS forms a G-quadruplex, it loses its ability to protect gold nanoparticles (AuNPs) from salt-induced AuNP aggregation. Therefore, the color of the solution changes from red to blue which can be visually observed. This colorimetric assay has a 0.13 nM detection limit and a wide linear range that extends from 5 nM to 1 μM.
Graphical abstract Schematic presentation of a colorimetric aptamer biosensor for adenosine detection based on DNA cycling amplification and salt-induced aggregation of gold nanoparticles.
We describe an electrochemical bioassay for the detection of the activity of methyltransferase (MTase), and for screening this enzyme’s inhibitors. The assay is based on the conjugation of a hemin to a G-quadruplex that enables enzymatic signal amplification with the aid of exonuclease III (ExoIII). In the first step, double-stranded DNA containing the quadruplex-forming oligomer is assembled on the surface of a gold electrode and then methylated by DNA adenine methyltransferase (DAM). After cleaved by endonuclease DpnI, the methylated DNA is digested by ExoIII and the quadruplex-forming oligomers are liberated. This leads to the formation of a hemin/G-quadruplex (in presence of hemin and of potassium ions). The hemin/G-quadruplex catalyzes the oxidization of hydroquinone by H2O2 and the benzoquinone was formed to generate electrochemical signal. Finally, the gold electrode modified with reduced graphene oxide was used as working electrode for performing differential pulse voltammetry. The method has a detection limit of 0.31 unit · mL−1. A study on the inhibition of MTase showed it was inhibited by epicatechin with an IC50 value of 157 μM.
We describe an electrochemical bioassay for the detection of the activity of methyltransferase and for screening for its inhibitors. Due to the conjugation of a hemin to a G-quadruplex, strong enzymatic signal amplification is enabled with the aid of exonuclease III.
Hetero-dimeric magnetic nanoparticles of the type Au-Fe3O4 have been synthesised from separately prepared, differently shaped (spheres and cubes), monodisperse nanoparticles. This synthesis was achieved by the following steps: (a) Mono-functionalising each type of nanoparticles with aldehyde functional groups through a solid support approach, where nanoparticle decorated silica nanoparticles were fabricated as an intermediate step; (b) Derivatising the functional faces with complementary functionalities (e.g. amines and carboxylic acids); (c) Dimerising the two types of particles via amide bond formation. The resulting hetero-dimers were characterised by high-resolution TEM, Fourier transform IR spectroscopy and other appropriate methods.
Graphical Abstract Nano-LEGO: Assembling two types of separately prepared nanoparticles into a hetero-dimer is the first step towards complex nano-architectures. This study shows a solid support approach to combine a gold and a magnetite nanocrystal.
An electrochemical nanoaptasensor is described that is based on the use of a glassy carbon electrode (GCE) modified with electrodeposited silver nanoparticles (AgNPs). An aptamer (Apt) against trinitrotoluene (TNT) was then immobilized on the AgNPs. The addition of TNT to the modified GCE leads to decrease in peak current (typically measured at a potential of ?0.45 V vs. Ag/AgCl) of riboflavin which acts as an electrochemical probe. Even small changes in the surface (as induced by binding of Apt to TNT) alter the interfacial properties. As a result, the LOD is lowered to 33 aM, and the dynamic range extends from 0.1 fM to 10 μM without sacrificing specificity.
Graphical abstract Schematic presentation of a nanoaptasensor which is based on a glassy carbon electrode (GCE) modified with electrodeposited silver nanoparticles (AgNPs) and aptamer (Apt). It was applied to the detection of 2,4,6-trinitrotoluene (TNT) with the help of riboflavin (RF) as a redox probe.
Nitrogen-doped graphene quantum dots (NGQDs) are shown to strongly enhance the integrated chemiluminescence (CL) of the permanganate-sulfite system. The mechanism of enhancement was investigated, and the catalytic effect of the NGQDs was proven. In contrast to other carbon-based nanomaterials, the enhancement by NGQDs is independent of particle size and surface. However, the pyridinic nitrogen on the surface of the NGQDs facilitates the transformation of dissolved oxygen into H2O2 and the generation of hydroxyl radicals. This induces the increase of CL intensity. However, in the presence of Fe3+, the nitrogen functions and phenol groups on the surface of the NGQDs will chelate it, and the CL signal is decreased as a result. This effect was used to design an assay for Fe3+ that has a wide response range (1?×?10?8 - 1?×?10?6 M) and a 4 nM detection limit. The method was successfully applied to the determination of Fe3+ in spiked real water samples.
Graphical abstract Nitrogen-doped graphene quantum dots (NGQDs) are demonstrated to strongly enhance the integrated chemiluminescence (CL) of the permanganate-sulfite system. The pyridinic N-atoms in NGQDs facilitate the transformation from dissolved oxygen into H2O2 and the generation of ?OH radicals. This leads to the highly enhanced CL of the system. In the presence of Fe3+, which will be chelated by the nitrogen functions and phenol groups on the surface of the NGQDs, the CL signal is decreased.
The authors describe a chemiluminescence (CL)-based assay for the determination of bromate. The method is based on the use of a solution of carbon quantum dots (CQDs) and sulfite. Strong CL (peak at 490 nm) is observed when bromate is injected into the solution. The CL increases linearly in the 0.3 to 10 μmol L?1 bromate concentration range, giving a 0.1 μmol L?1 limit of detection (at an S/N ratio of 3). A possible CL mechanism is suggested that involves a redox reaction between the CQDs, bromate and sulfite in the acidic medium. This leads to the formation of hole-injected and electron-injected CQDs. Radiative recombination of oxidant-injected holes and electrons in the CQDs accounts for the occurrence of CL. This mechanism contradicts the previous assumption that the transfer of energy occurs from SO2* to the CQDs. Although nitrite may interfere in the determination of bromate, its effect can be eliminated by adding sulfamic acid. The assay is sensitive and represents a new tool for the determination of bromate, which is a carcinogen.
Graphical abstract Under acidic condition, carbon quantum dots (CQDs) can react with sulfite and bromate transforming to hole-injected CQDs (CQDs?–) and electron-injected CQDs (CQDs?+), respectively. Thereafter, strong chemiluminescence (490 nm) aroused from the radiative electron-hole annihilation between CQDs?– and CQDs?+.
The authors describe an upconversion nanoparticle-based (UCNP–based) fluorometric method for ultrasensitive and selective detection of Cu2+. The UCNPs show a strong emission band at 550 nm under near-infrared excitation at 980 nm. The principle of the strategy is that gold nanoparticles (AuNP) can quench the fluorescence of UCNP. In contrast, the addition of L-cysteine (Cys) can induce the aggregation of AuNP, resulting in a fluorescence recovery of the UCNPs. On addition of Cu2+, it oxidizes Cys to cystine and is reduced to Cu+. The Cu+ thusformed can be oxidized cyclically to Cu2+ by dissolved O2, which catalyzes and recycles the whole reaction. Thus, the aggregation of AuNP is inhibited and the fluorescence recovered by Cys is quenched. Under the optimal condition, the quenching efficiency shows a good linear response to the concentrations of Cu2+ in the 0.4–40 nM range. The limit of detection is 0.16 nM, which is 5 orders of magnitude lower than the U.S. Environmental Protection Agency limit for Cu2+ in drinking water (20 μM). The method has been further applied to monitor Cu2+ levels in real samples. The results of detection are well consistent with those obtained by atomic absorption spectroscopy.
Graphical abstract Gold nanoparticles (AuNP) as a high efficient fluorescence quenching reagent of upconversion nanoparticles (UCNP) were used in a fluorometric method for detection of Cu2+ based on a cyclic catalytic oxidation amplification strategy.
The authors describe an electrochemical sensor for the breast cancer marker α-lactalbumin (αLA). It is based on the use of printed single-walled carbon nanotube (SWCNT) electrodes that were modified with polycatechol. Impedance-derived electrochemical capacitance spectroscopy (ECS) is applied for detection at an applied potential of ?0.14 V vs. Ag/AgCl reference electrode. The electrode was prepared in a two-step process. First, a dispersion of SWCNTs was drop-cast onto the surface of a poly(ethylene terephthalate) substrate to act as the working electrode. Next, catechol was electrochemically polymerized on the SWCNTs, prior to the immobilization of lysozyme. The strong interaction between lysozyme and αLA induced changes in the redox capacitance which are detected by ECS. The latter shows the device to be capable of detecting αLA in the 20 to 80 ng·mL?1 concentration range. The limit of detection is 9.7 ng·mL?1 at an S/N ratio of 3. The device was used to detect αLA in human blood serum with good recovery results.
Graphical abstract A sensitive biosensor for αLA was prepared by modifying SWCNT electrode with polycatechol and lysozyme. The electrochemical capacitance spectroscopy was used for the first time to selectively detect αLA in the blood in the range from 20 to 80 ng·mL?1.
We report on a highly sensitive competitive immunoassay for the mycotoxin Ochratoxin (OTA) using magnetic silica nanoparticles (NPs) fluorescently labeled with rhodamine 123 (Rho123) as signal intensifier. The method is based on the measurement of fluorescence resonance energy transfer (FRET) that occurs from CdTe quantum dots covered with anti-OTA antibody to the dye Rho123 on the surface of the NPs. The immunoreaction between anti-OTA antibody and OTA brings the fluorophore (acting as the acceptor) in close proximity of the QDs (acting as the donor), and this causes FRET to occur upon photo-excitation of the QDs. The size and polydispersity of the silica coated magnetic NPs was studied via TEM. The method has a detection limit of 0.8 pg of OTA per mL. It was applied to the determination of OTA in spiked human serum. A linear relationship is found between the increase in the fluorescence intensity of Rho 123 at 580 nm and the concentration of OTA in spiked samples over the 8 to 48 pg?mL?1 concentration range. This highly sensitive homogeneous competitive detection scheme is simple, rapid and efficient. It does not require multiple separation steps and excessive washing.
Graphical abstract Following photoexcitation of immobilized quantum dots (QDs), FRET occurs between the QDs and Rhodamine 123. The close proximity of Rho 123 and the magnetic silica core/shell particles leads to strongly intensified emission to result in an assay for Ochratoxin A that has a detection limit as low as 0.8 pg?mL-1
The authors describe four different kinds of sorbents for solid-phase extraction (SPE) and preconcentration of proteins from complex samples. All are based on the use of a poly(glycidyl-co-ethylene dimethacrylate) host monolith that was chemically functionalized by using two different ligands (ammonia and cysteamine). Gold nanoparticles (AuNPs) or silver NPs were then assembled to the amino or thiol groups. The resulting materials are shown to be viable stationary phases for use in SPE cartridges. The sorbents can selectively retain bovine serum albumin, and the thiol-modified sorbents containing AuNPs and AgNPs provide the highest recoveries (>90%) and satisfactory loading capacities (29.3 and 17.6 μg?mg?1 of sorbent, respectively). The applicability of these nanosorbents was demonstrated by preconcentrating viscotoxins from mistletoe extracts. The enriched fractions were subjected to MALDI-TOF analysis to underpin their selectivity.
Graphical abstract Hybrid materials based on methacrylate polymers modified with gold or silver nanoparticles were used as sorbents for solid phase extraction and preconcentration of bovine serum albumin and mistletoe viscotoxins, this followed by MALDI-TOF analysis.
The authors describe an SPR sensor chip coated with gold nanoparticles (AuNPs) that enables highly sensitive determination of genetically modified (GM) crops. Detection is based on localized surface plasmon resonance (LSPR) with its known sensitivity to even minute changes in refractive index. The device consists of a halogen light source, a light detector, and a cuvette cell that contains a sensor chip coated with AuNPs. It is operated in the transmission mode of the optical path to enhance the plasmonic signal. The sample solution containing target DNA (e.g. from the GM crop) is introduced into the cuvette with the sensor chip whose surface was functionalized with a capture DNA. Following a 30-min hybridization, the changes of the signal are recorded at 540 nm. The chip responds to target DNA in the 1 to 100 nM concentration range and has a 1 nM detection limit. Features of this sensor chip include a short reaction time, ease of handling, and portability, and this enables on-site detection and in-situ testing.
Graphical abstract A localized surface plasmon resonance (LSPR)-based nanoplasmonic spectroscopic device enabling a highly sensitive biosensor is developed for the detection of genetically modified (GM) DNA founded in Roundup Ready (RR) soybean.
Charge tagging is a peptide derivatization process that commonly localizes a positive charge on the N-terminus. Upon low energy activation (e.g., collision-induced dissociation or post-source decay) of charge tagged peptides, relatively few fragment ions are produced due to the absence of mobile protons. In contrast, high energy fragmentation, such as 157 nm photodissociation, typically leads to a series of a-type ions. Disadvantages of existing charge tags are that they can produce mobile protons or that they are undesirably large and bulky. Here, we investigate a small triethylphosphonium charge tag with two different linkages: amide (158 Da) and amidine bonds (157 Da). Activation of peptides labeled with a triethylphosphonium charge tag through an amide bond can lead to loss of the charge tag and the production of protonated peptides. This enables low intensity fragment ions from both the protonated and charge tagged peptides to be observed. Triethylphosphonium charge tagged peptides linked through an amidine bond are more stable. Post-source decay and photodissociation yield product ions that primarily contain the charge tag. Certain amidine induced fragments are also observed. The previously reported tris(trimethoxyphenyl) phosphonium acetic acid N-hydroxysuccinimidyl ester charge tag shows a similar fragment ion distribution, but the mass of the triethylphosphonium tag label is 415 Da smaller.
