A simple and cost-effective fluorescence immunoassay for the sensitive quantitation of disease biomarker α-fetoprotein (AFP) has been developed based on the phosphate-triggered fluorescence turn-on ...detection of alkaline phosphatase (ALP), with the reversible binding between calcein and Ce3+ as a signaling element. In this immunoassay, fluorescent calcein is readily quenched by Ce3+ via a coordination process. The ALP-catalyzed hydrolysis of p-nitrophenyl phosphate leads to the formation of p-nitrophenol and inorganic orthophosphate, and the newly formed orthophosphate could potently combine with Ce3+ due to the higher affinity, thus, recovering the fluorescence of calcein. The corresponding fluorescence signal triggered by phosphate is related to ALP activities labeled on antibody, and thus could be applied to detect target antigen in an enzyme-linked immunosorbent assay (ELISA) platform. The fluorescence intensity correlated well to the AFP concentration ranges of 0.2–1.0 and 1.0–4.0 ng/mL, with a detection limit of 0.041 ng/mL. The proposed fluorescence ELISA possesses convincing recognition mechanism and exhibits excellent assay performance in the evaluation of the AFP level in serologic test, which unambiguously reveals great application potential in the clinic diagnosis of disease biomarkers.
A novel, highly sensitive and selective dual-readout (colorimetric and fluorometric) sensor based on fluorescent carbon dots (CDs) and unmodified gold nanoparticles (AuNPs) for the detection of ...thiocyanate (SCN(-)) was proposed. Amino-functionalized CDs could be readily adsorbed onto the surface of citrate-stabilized AuNPs through Au-N interactions, leading to the aggregation of AuNPs and the nonfluorescent off-state of CDs arising from potential fluorescence resonance energy transfer (FRET). However, SCN(-) had a stronger affinity toward AuNPs and could compete with CDs to bind onto the surface of AuNPs in priority, which prevented the aggregation of AuNPs and fluorescence quenching of CDs. Correspondingly, both the colorimetric and fluorometric signals remained "light-on". The color of the sensing solution remained red and the fluorescence remained unquenched. A distinguishable change in the color was observed at a SCN(-) concentration of 1 μM by the naked eye and a detection limit as low as 0.036 μM was obtained by virtue of fluorescence spectroscopy. Both colorimetric and fluorometric sensors exhibited excellent selectivity toward SCN(-) over other common metallic ions and anions. In addition, such a sensing assay featured simplicity, rapidity, cost-effectiveness and ease of operation without further modification. The accuracy and precision were evaluated based on the quantitative detection of SCN(-) in tap water and saliva samples with satisfactory results.
Core-shell palladium cube@CeO
2
(Pd cube@CeO
2
) nanoparticles are shown to display oxidase-like activity. This is exploited in a method for determination of the activity of alkaline phosphatase ...(ALP). The Pd cube@CeO
2
nanoparticles were thermally synthesized from Ce(NO
3
)
3
, L-arginine and preformed Pd cube seeds in water. The Pd cube@CeO
2
nanoparticles catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by oxygen. This results in the formation of oxidized TMB (oxTMB) with an absorption peak at 652 nm. Ascorbic acid (AA) is generated from the hydrolysis of L-ascorbic acid 2-phosphate (AAP) catalyzed by ALP. It can reduce oxTMB to TMB, and this results in a decrease of the absorbance. The method allows for quantitative determination of the activity of ALP in the range from 0.1 to 4.0 U·L
−1
and with a detection limit down to 0.07 U·L
−1
. Endowed with high sensitivity and selectivity, the assay can quantify ALP activity in biological system with satisfactory results.
Graphical abstract
Schematic illustration of Pd cube@CeO
2
core-shell nanoparticles for colorimetric determination of alkaline phosphatase.
A simple label-free colorimetric sensor for sulfate has been devised based on the analyte-induced shielding of peroxidase-like activity of cysteamine-capped gold nanoparticles (cyst-AuNPs). ...Aggregation triggered by sulfate through hydrogen bond and electrostatic interaction gave rise to a remarkable decrease of the catalytic activity of AuNPs. By coupling with 3,3′,5,5′-tetramethylbenzidine (TMB)–H2O2 chromogenic reaction, a novel colorimetric sensor for detection of sulfate was established. Based on the signal amplification effect of the catalytic reaction, a detection limit as low as 0.16μM was obtained by virtue of UV–vis spectroscopy or 4μM with the naked-eye observation. The as-proposed sensor exhibited an excellent selectivity over other common anions and metallic ions. In view of these characteristics, such sensing probe features simplicity, cost-effectiveness, and naked-eye screening. These advantages make this sensor a powerful protocol for the quantitative detection of sulfate in water samples with satisfactory results.
Enzyme-mimetic properties of nanomaterials can be efficiently tuned by controlling their size, composition, and structure. Here, ultrathin PdCu alloy nanosheet–assembled three-dimensional (3D) ...nanoflowers (Pd
1
Cu
x
NAFs) with tunable surface composition are obtained via a generalized strategy. In presence of H
2
O
2
, the as-synthesized Pd
1
Cu
x
NAFs can catalyze the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to the oxidized form of TMB (oxTMB) with a characteristic absorption peak at 652 nm. Interestingly, Pd
1
Cu
x
NAFs show obviously composition-dependent peroxidase-like catalytic activities because of the synergistic interaction of nanoalloy. Additionally, different from 2D Pd nanosheets, the distinctive 3D superstructures are featured with rich approachable sites and proper layer spacing, which are in favor of fast mass transport and electron transfers during the catalytic process. Among the studied Pd
1
Cu
x
NAFs, the Pd
1
Cu
1.7
NAFs show the highest enzyme-like activities and can be successfully applied for the colorimetric detection of glucose with a low detection limit of 2.93 ± 0.53 μM. This work provides an efficient avenue to fabricate PdCu NAF nanozymes in biosensing toward glucose detection.
Graphical abstract
Two-dimensional (2D) PdCu ultrathin nanosheet–assembled 3D nanoflowers (Pd
1
Cu
x
NAFs) with tunable surface composition exhibit substantially enhanced intrinsic peroxidase-like catalytic activities. The Pd
1
Cu
1.7
NAFs are successfully used as peroxidase mimic catalyst for the colorimetric detection of glucose with low detection limit of 2.93 μM.
The undesirable enzymatic activity of nanozymes under near neutral pH condition and the traditional single signal output always restrict the analytical application of nanozyme-based biosensors. ...Herein, graphitic carbon nitride nanosheets supported palladium nanosheets composite (Pd/g-C3N4) with both oxidase-like activity and fluorescent property is synthesized. Notably, Pd/g-C3N4 exhibits enhanced oxidase-like activity compared to Pd NSs under pH 7.4. By combining Pd/g-C3N4 with o-phenylenediamine (OPD), a ratiometric fluorescence assay for acetylcholinesterase (AChE) activity detection is developed. Pd/g-C3N4 can catalyze oxidation of nonfluorescent OPD to fluorescent oxidized OPD (oxOPD, Em = 565 nm), which can quench fluorescence of g-C3N4 supporter (Em = 441 nm) through fluorescence resonance energy transfer (FRET). However, in presence of AChE, acetylthiocholine can be hydrolyzed into thiocholine, which will block the oxidase-like activity of Pd/g-C3N4 and then hamper the FRET process. This ratiometric fluorescence assay is also viable to screen AChE inhibitor. This work will guide design of ratiometric fluorescence assay based on nanozymes with improved enzymatic activity.
A Pd/g-C3N4 nanocomposite with improved oxidase-like activity compared to Pd nanosheets is synthesized and used to develop a ratio fluorescence strategy for detection of acetylcholinesterase (AChE) activity and its inhibitor. In this assay, the oxidase mimetic activity of Pd/g-C3N4 can be inhibited via the coordination interaction of Pd and thiol-containing thiocholine (TCh), which is hydrolyzed from acetylthiocholine (ATCh) under the catalysis of specific AChE. Display omitted
A convenient and sensitive colorimetric assay for acetylcholinesterase (AChE) and its inhibitor has been designed based on the oxidase-like activity of {100}-faceted Pd square nanoplates which are ...grown in situ on reduced graphene oxide (PdSP@rGO). PdSP@rGO can effectively catalyze the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) without the assistance of H
2
O
2
to generate blue oxidized TMB (oxTMB) with a characteristic absorption peak at 652 nm. In the presence of AChE, acetylthiocholine (ATCh), a typical AChE substrate, is hydrolyzed to thiocholine (TCh). The generated TCh can effectively inhibit the PdSP@rGO-triggered chromogenic reaction of TMB via cheating with Pd, resulting in color fading and decrease in absorbance. Thus, a sensitive probe for AChE activity is constructed with a working range of 0.25–5 mU mL
−1
and a limit of detection (LOD) of 0.0625 mU mL
−1
. Furthermore, because of the inhibition effect of tacrine on AChE, tacrine is also detected through the colorimetric AChE assay system within the concentrations range 0.025–0.4 μM with a LOD of 0.00229 μM. Hence, a rapid and facile colorimetric procedure to sensitively detect AChE and its inhibitor can be anticipated through modulating the oxidase-like activity of PdSP@rGO.
Graphical abstract
Colorimetric method for detection of AChE and its inhibitor is established by modulating the oxidase mimetic activity of {100}-faceted Pd square nanoplates on reduced graphene oxide (PdSP@rGO).
The authors describe a fluorometric method for determination of the activity of alkaline phosphatase (ALP) and its inhibitors. Nitrogen and boron co-doped carbon dots (C-dots) with ...excitation/emission peaks at 490/540 nm act as the fluorescent probe. The C-dots were prepared by hydrothermal carbonization starting from 3-aminophenylboronic acid as the sole precursor. On the basis of the boronic acid-triggered specific reaction with
cis
-diols, the boronic acid modified C-dots can bind to ascorbic acid that is generated by ALP-catalyzed hydrolysis of ascorbic acid 2-phosphate. This results in particle aggregation and quenching of fluorescence. If the ALP inhibitor Na
3
VO
4
is introduced into the system, the activity of ALP is reduced and the fluorescence of C-dots recovers. This fluorometric method allows for the determination of ALP activity in the range from 0.2 to 6.0 mU mL
−1
with a detection limit of 0.16 mU mL
−1
. The IC
50
value for the inhibitor Na
3
VO
4
is 3.6 μM. The method is convenient and cost-effective. It does not require complicated operations and in our perception widens the scope of applications of C-dots in bioanalytical sciences.
Graphical abstract
Schematic presentation of the nitrogen and boron co-doped carbon dot-based fluorometric method for determination of alkaline phosphatase (ALP) activity.
An ultrasensitive fluorometric and colorimetric dual-mode assay is described for the determination of the activity of alkaline phosphatase (ALP). ALP catalyzes the decomposition of ...2-phospho-L-ascorbic acid, and the ascorbic acid thus generated reduces silver ions. In the presence of gold nanoparticles, gold-silver nanoparticles (Au@Ag NPs) are formed. This is accompanied by a color change form pink to deep yellow. The Au@Ag NPs reduce the fluorescence of blue fluorescent graphene quantum dots due to spectral overlap. The changes of absorbance (measured at 410 and 520 nm) and fluorescence (measured at excitation/emission wavelengths of 346/415 nm) correlate well with the ALP activity in the 0.01–6 mU·mL
−1
(absorption) and 0.01–2 mU·mL
−1
(fluorescence) ranges, and the detection limits are 9 and 5 μU·mL
−1
individually.
Graphical abstract
Schematic presentation of colorimetric and fluorometric dual-readout assay for alkaline phosphatase (ALP) activity. It is based on enzymatically induced formation of gold-silver nanoparticles (Au@Ag NPs), and the fluorescence quenching of graphene quantum dots due to inner filter effect.
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