**2.4 Nanocomposites based on graphene quantum dots (GQDs) as electrochemical sensors**

The functionalized glassy carbon electrode (GCE) based on composites of GQDs and β-cyclodextrins (β-CDs) was designed and synthesized as an electrochemical sensor for the detection of tyrosine (Tyr) enantiomers [35]. The as-synthesized β-CDs-GQDs/GCE exhibited an ultrasensitive response signal for the monitoring of Tyr enantiomers using GQDs as substrate and β-CDs as recognition molecule. The β-CDs-GQDs/GCE showed obvious difference in the oxidation peak current between l-Tyr and d-Tyr. The quantities of l-Tyr of healthy people showed higher than that of depression patients. The detection limit was 6.07 × 10<sup>−</sup><sup>9</sup> M and 1.03 × 10<sup>−</sup><sup>7</sup> M for l-Tyr and d-Tyr, respectively.

The gold nanoparticles/proline-functionalized GQDs (GNs/Pro-GQDs) were constructed and prepared as ultrasensitive electrochemical sensor for the monitoring of p-acetamidophenol [36]. The proline-GQDs were synthesized using pyrolysis of citric acid and proline. The GNs/Pro-GQDs were formed by directly reacting HAuCl4 with proline-GQDs. The peak current (*I*p) showed a good linear relationship with the concentration of p-acetamidophenol within 0.08–100 mM. The detection limit was 0.02 μM (S/N = 3).

The GQDs/riboflavin (RF) functionalized glassy carbon elec-trode (GC/ GQDs/RF) was developed as a sensitive electrochemical sensor to detect persulfate (S2O8 <sup>2</sup><sup>−</sup>) [37]. The modified electrode exhibited a stable redox peak between pH 1

**183**

*Nanocomposite-Based Graphene for Nanosensor Applications*

and pH 10. The obtained GC/GQDs/RF showed a good electrochemical activity for

mechanism of fluorescence resonance energy transfer (FRET). The linear response range was 50 nM–60 μM. The detection limit was 20 nM (S/N = 3). The obtained

The hybrid GQDs/TiO2 NTs were constructed based on titanium dioxide nanotube arrays (TiO2 NTs) infilled with GQDs as an efficient ECL sensor for detection of PSA [39]. The fabricated GQDs/TiO2 NP composite electrode presented good stability and showed higher fluorescence intensity compared to that of pure TiO2 NT electrode. The TiO2 functionalized Fe3O4 magnetic nanoparticles (CdTe/MNPs) acted as quencher for the sensor. The GQDs/TiO2 NT sensing platform showed high sensitivity and high selectivity for the detection of PSA. The ECL quenching efficiency exhibited a good linear relationship with log of the concentration of the PSA within 1.0 fg/mL to 10 pg/mL. The detection limit was 1 fg/mL (S/N = 3). The obtained nanosensor has been successfully applied to detect PSA in clinical human serum samples. The label-free ECL immunosensor was designed and synthesized based on GQDs [40]. The Au/Ag-rGO complex was prepared and employed to immobilize GQDs. The aminated-GQDs and carboxyl-GQDs were loaded onto electrode. The antibody of PSA was conjugated with modified electrode by absorbing Au/Ag to target proteins. The ECL quenching efficiency showed a linear relationship with log of concentrations between 1 pg/mL to 10 ng/mL. The detection limit

The chitosan-functionalized GQDs (GQD-CS) were constructed and employed

The GQDs coated on hollow nickel nanospheres (hNiNS) modified with glass carbon electrode (GCE) were designed and synthesized as a molecularly imprinted electrochemical sensor (MIECS) for the monitoring of bisphenol S (BPS) with high sensitivity and high selectivity [42]. The pyrrole serves as monomer and BPS as template to polymerized molecularly imprinted polymer (MIP) film. The response signal showed linear relationship with the concentration of BPS between 0.1 and 50 μM. The detection limit was 0.03 μM. The ultrasensitive electrochemical sensor based on modified glass carbon electrode (GCE) was constructed and prepared for the determination of metronidazole (MNZ) [43]. The GQDs coated with molecularly imprinted polymers (MIPs) were synthesized. The complex of graphene nanoplatelets (GNPs) and MIPs exhibited obviously enhanced electrocatalytic property for MNZ based on good synergistic effect of GNPs and MIPs. The proposed electrochemical sensor displayed two linear ranges within 0.005– 0.75 μmol/L and 0.75–10.0 μmol/L. The detection limit was 0.52 nmol/L. The electrochemical sensor has been applied to inspection of human serum samples. The GQD self-assembled monolayer-modified electrode was constructed as highly selective electrochemical sensor for the detection of dopamine (DA) [44]. The GQD-NHCH2CH2NH2 functionalized GCE was prepared. The functionalized

to mobilize methylene blue (MB) using glass carbon electrode (GCE) based on aminohydroxy reaction [41]. The non-enzymatic sensor showed high sensitivity and high selectivity for the detection of H2O2. The obtained GQD-CS/MB/GCE displayed an obviously catalytic behavior toward H2O2 reduction. Compared with bare GCE, GQDs/GCE, and GQD-CS/GCE, the hybrid GQD-CS/MB/GCE showed higher electrochemical activities based on synergistic effect between GQD-CS and

MB. The sensitivity was 10.115 μA/mM and detection limit was 0.7 μM.

detection limit and sensitivity were 0.2 μM and 4.7 nA/μM, respectively. One electrochemiluminescent (ECL) sensor was developed and synthesized to monitor Cr(VI) ions in water samples based on fluorescence signal changes of graphene

sensor has been successfully applied to detect Cr(VI) in river water.

<sup>2</sup><sup>−</sup>. The linear calibration range was from 1.0 μM to 1 mM. The

<sup>2</sup><sup>−</sup> complex was quenched in the presence of Cr(VI) ions based on

<sup>2</sup><sup>−</sup>) complex [38]. The fluorescence

*DOI: http://dx.doi.org/10.5772/intechopen.85136*

quantum dots/peroxodisulfate (GQD/S2O8

the detection of S2O8

of GQD/S2O8

was 0.29 pg/mL.

*Nanorods and Nanocomposites*

detection limit was 0.192 pg/mL.

**electrochemical sensors**

detection limit was 0.02 μM (S/N = 3).

successfully applied to monitor ACP in PC-3 M cells.

lecithin/β-cyclodextrin (lecithin/β-CD) complex as linker [32]. The GQDs connect with NR through lecithin/β-CD complex based on electrostatic interaction and hydrophobic interaction. The fluorescence of GQDs was quenched upon addition of lecithin/β-CD@NR based on Förster resonance energy transfer. Meantime, the fluorescence intensity of NR obviously enhanced. The lecithin/β-CD@NR@ GQD complex as nanosensor exhibited high sensitivity for the detection of acid phosphatase (ACP). The detection limit was 28 μU/mL. The proposed sensor has been

The graphene oxide quantum dots@silver (GQDs@Ag) nanocrystals with core-shell structure was designed and prepared as fluorescence sensing platform for the detection of prostate-specific antigen (PSA) [33]. The quantities of GQDs on GQDs@Ag decided the intensities of fluorescence signal. The incorporated GQDs can be released by removing of silver shell based on oxidative reaction without affecting their fluorescence performance. The anti-PSA antibody (Ab1) and antibody (Ab2) was immobilized onto magnetic beads (MBs) and GQDs@Ag, respectively. The GQDs@Ag showed a high sensitivity and high selectivity for the detection of PSA. The fluorescence intensity exhibited an excellent linear relationship with concentrations of PSA within 1 pg/mL to 20 ng/mL. The detection limit was 0.3 pg/ mL. The as-synthesized immunosensor has been successfully applied to detect PSA in human serum. The antibody anti-cardiac troponin I (anti-cTnI) modified with amine-functionalized GQDs (afGQDs) was constructed and prepared by carbodiimide coupling reaction, namely, anti-cTnI/afGQDs [34]. The complex anti-cTnI/ afGQDs exhibited sensitive response for detection of target antigen (cTnI) with high sensitivity and high selectivity. The as-synthesized complex as nanosensor showed strong fluorescence behavior, and the fluorescence of anti-cTnI/afGQDs was quenched in the presence of graphene (Gr). The fluorescence of anti-cTnI/afGQDs was recovered upon the addition of target antigen (cTnI) on anti-cTnI/afGQDs/ Gr-inducing Gr apart from GQDs. The fluorescence intensity showed a good linear relationship with the concentrations of cTnI between 1.0 pg/mL and 1.0 ng/mL. The

**2.4 Nanocomposites based on graphene quantum dots (GQDs) as** 

M for l-Tyr and d-Tyr, respectively.

The functionalized glassy carbon electrode (GCE) based on composites of GQDs and β-cyclodextrins (β-CDs) was designed and synthesized as an electrochemical sensor for the detection of tyrosine (Tyr) enantiomers [35]. The as-synthesized β-CDs-GQDs/GCE exhibited an ultrasensitive response signal for the monitoring of Tyr enantiomers using GQDs as substrate and β-CDs as recognition molecule. The β-CDs-GQDs/GCE showed obvious difference in the oxidation peak current between l-Tyr and d-Tyr. The quantities of l-Tyr of healthy people showed higher than that of depression patients. The detection limit was 6.07 × 10<sup>−</sup><sup>9</sup>

The gold nanoparticles/proline-functionalized GQDs (GNs/Pro-GQDs) were constructed and prepared as ultrasensitive electrochemical sensor for the monitoring of p-acetamidophenol [36]. The proline-GQDs were synthesized using pyrolysis of citric acid and proline. The GNs/Pro-GQDs were formed by directly reacting HAuCl4 with proline-GQDs. The peak current (*I*p) showed a good linear relationship with the concentration of p-acetamidophenol within 0.08–100 mM. The

The GQDs/riboflavin (RF) functionalized glassy carbon elec-trode (GC/ GQDs/RF) was developed as a sensitive electrochemical sensor to detect persulfate

<sup>2</sup><sup>−</sup>) [37]. The modified electrode exhibited a stable redox peak between pH 1

M and

**182**

(S2O8

1.03 × 10<sup>−</sup><sup>7</sup>

and pH 10. The obtained GC/GQDs/RF showed a good electrochemical activity for the detection of S2O8 <sup>2</sup><sup>−</sup>. The linear calibration range was from 1.0 μM to 1 mM. The detection limit and sensitivity were 0.2 μM and 4.7 nA/μM, respectively. One electrochemiluminescent (ECL) sensor was developed and synthesized to monitor Cr(VI) ions in water samples based on fluorescence signal changes of graphene quantum dots/peroxodisulfate (GQD/S2O8 <sup>2</sup><sup>−</sup>) complex [38]. The fluorescence of GQD/S2O8 <sup>2</sup><sup>−</sup> complex was quenched in the presence of Cr(VI) ions based on mechanism of fluorescence resonance energy transfer (FRET). The linear response range was 50 nM–60 μM. The detection limit was 20 nM (S/N = 3). The obtained sensor has been successfully applied to detect Cr(VI) in river water.

The hybrid GQDs/TiO2 NTs were constructed based on titanium dioxide nanotube arrays (TiO2 NTs) infilled with GQDs as an efficient ECL sensor for detection of PSA [39]. The fabricated GQDs/TiO2 NP composite electrode presented good stability and showed higher fluorescence intensity compared to that of pure TiO2 NT electrode. The TiO2 functionalized Fe3O4 magnetic nanoparticles (CdTe/MNPs) acted as quencher for the sensor. The GQDs/TiO2 NT sensing platform showed high sensitivity and high selectivity for the detection of PSA. The ECL quenching efficiency exhibited a good linear relationship with log of the concentration of the PSA within 1.0 fg/mL to 10 pg/mL. The detection limit was 1 fg/mL (S/N = 3). The obtained nanosensor has been successfully applied to detect PSA in clinical human serum samples. The label-free ECL immunosensor was designed and synthesized based on GQDs [40]. The Au/Ag-rGO complex was prepared and employed to immobilize GQDs. The aminated-GQDs and carboxyl-GQDs were loaded onto electrode. The antibody of PSA was conjugated with modified electrode by absorbing Au/Ag to target proteins. The ECL quenching efficiency showed a linear relationship with log of concentrations between 1 pg/mL to 10 ng/mL. The detection limit was 0.29 pg/mL.

The chitosan-functionalized GQDs (GQD-CS) were constructed and employed to mobilize methylene blue (MB) using glass carbon electrode (GCE) based on aminohydroxy reaction [41]. The non-enzymatic sensor showed high sensitivity and high selectivity for the detection of H2O2. The obtained GQD-CS/MB/GCE displayed an obviously catalytic behavior toward H2O2 reduction. Compared with bare GCE, GQDs/GCE, and GQD-CS/GCE, the hybrid GQD-CS/MB/GCE showed higher electrochemical activities based on synergistic effect between GQD-CS and MB. The sensitivity was 10.115 μA/mM and detection limit was 0.7 μM.

The GQDs coated on hollow nickel nanospheres (hNiNS) modified with glass carbon electrode (GCE) were designed and synthesized as a molecularly imprinted electrochemical sensor (MIECS) for the monitoring of bisphenol S (BPS) with high sensitivity and high selectivity [42]. The pyrrole serves as monomer and BPS as template to polymerized molecularly imprinted polymer (MIP) film. The response signal showed linear relationship with the concentration of BPS between 0.1 and 50 μM. The detection limit was 0.03 μM. The ultrasensitive electrochemical sensor based on modified glass carbon electrode (GCE) was constructed and prepared for the determination of metronidazole (MNZ) [43]. The GQDs coated with molecularly imprinted polymers (MIPs) were synthesized. The complex of graphene nanoplatelets (GNPs) and MIPs exhibited obviously enhanced electrocatalytic property for MNZ based on good synergistic effect of GNPs and MIPs. The proposed electrochemical sensor displayed two linear ranges within 0.005– 0.75 μmol/L and 0.75–10.0 μmol/L. The detection limit was 0.52 nmol/L. The electrochemical sensor has been applied to inspection of human serum samples. The GQD self-assembled monolayer-modified electrode was constructed as highly selective electrochemical sensor for the detection of dopamine (DA) [44]. The GQD-NHCH2CH2NH2 functionalized GCE was prepared. The functionalized

electrode showed excellent electrical conductivity and displayed sensitive response to DA. The modified GCE showed a good linear relationship with the concentrations within 1–150 μM. The detection limit was 0.115 μM (S/N = 3). The obtained GQD-NHCH2CH2NH2 functionalized GCE displayed good stability and excellent anti-interference capability.

### **2.5 Nanocomposite-based doped graphene quantum dots as nanosensors**

The chemical doping is a common strategy and used for tailoring the properties of GQDs. The heteroatom-doped GQDs showed exceptional properties such as tunable emission, changeable spin density, and charge distribution of carbon atoms [45]. Dopants include N, sulfur (S), phosphorus (P), boron (B), fluorine (F), and chlorine (Cl).

### *2.5.1 Nitrogen-doped graphene quantum dots (N-GQDs) as nanosensors*

The first successful synthesis of nitrogen-doped GQDs was reported by Li and co-workers in 2012 [46]. Liu et al. synthesized N-GQDs by hydrothermal method using citric acid as carbon sources and ammonia as nitrogen sources with N/C atomic ratio of ca. 4.3% emitting an obviously blue luminescence [47]. The fluorescence quantum yield of N-GQDs was 2.46% by calculation. The as-prepared N-GQDs can strongly adsorb 18 mer ssDNA (5′-ATACCAGCTTATTCAATT-3′) via π-π interaction force. The fluorescence of N-GQDs was quenched by photo-induced electron transfer mechanism between N-GQDs and ssDNA. The fluorescence of N-GQDs can be recovered upon addition of mixture of bleomycin and Fe(II) due to the noncovalent binding between bleomycin and ssDNA.

Fan and co-worker constructed N-GQD-Hg(II) complex system as a highly sensitive fluorescence sensor for cysteine detection [48]. The N-GQDs was prepared by one-pot method using citric acid as carbon source and urea as nitrogen sources. The N-GQD-Hg(II) complex as fluorescence sensor showed weak fluorescence. The fluorescence was recovered upon addition of cysteine to the complex system of N-GQD-Hg(II) due to the coordinate interaction between cysteine and Hg(II). The fluorescence intensity showed good linear relationship with the concentration of cysteine within a range of 0.05–30 μmol/L. The detection limit was 1 .3 nmol/L.

Zhao et al. prepared oxygen-rich nitrogen-doped GQDs by using one-pot synthesis strategy as pH-sensitive sensor for the detection of Hg(II) ion [49]. The oxygen-rich N-GQDs were synthesized by using citric acid (CA) and 3,4-dihydroxyl-phenylalanine (l-DOPA) as the carbon source and the N source, respectively. The N-GQDs showed excitation-wavelength-independent fluorescent behavior, and the quantum yield was 18%. The N-GQDs as an efficient fluorescent sensor displayed the highly sensitivity and highly selectivity for the detection of Hg(II) based on the mechanism of nonradiative electron transfer. The detection limit was 8.6 nM. The fluorescence quenching efficiency showed good linear relationship with the concentration of Hg(II) within concentration from 0.04 to 6 μM. The competitive experiments showed that the N-GQDs showed high selectivity and sensitivity for the detection of Hg(II) even in the interference of other metal ions.

The strip-based fluorescence molecularly imprinted sensor was designed and constructed for monitoring thiacloprid [50]. The fluorescence molecularly imprinted sensor was synthesized based on polydopamine (PDA) polymer, thiacloprid, and N-GQDs. Firstly, the filter paper is dipped into N-GQD aqueous solution; secondly, the dopamine with thiacloprid self-polymerized on the surface of strip. The polydopamine molecularly imprinted polymer acted as an high efficient

**185**

of sp3

*Nanocomposite-Based Graphene for Nanosensor Applications*

good biocompatibility of the NGQD@NC@Pd/GCE.

sensor for the detection of thiacloprid. The as-prepared fluorescence molecularly imprinted sensor showed a linear relationship between 0.1 and 10 mg/L, and detec-

Peng and co-workers designed and reported a strategy method to detect Hg(II) ions by accelerating reaction rate between porphyrin and Mn(II) based on synergistic effect of N-GQDs and Hg(II) [53]. The reaction mechanism is based on larger Hg(II) of porphyrin-Hg(II) complex, which was replaced by smaller Mn(II) ions forming porphyrin-Mn(II) complex in a relatively faster speed. Such course was accompanied by the absorption red-shift and fluorescence quenching of porphyrins; meantime, the fluorescence intensity of N-GQDs enhanced. The CCK-8 assay showed over 90% viability by incubating 5.0 μM TMPyP, 40 μM Mn(II), or 20 μg/L N-GQDs for 24 h using A549 cells, indicative of good

*2.5.2 Nitrogen and phosphorus co-doped graphene quantum dots (N,P-GQDs) as* 

of N,P-GQDs was 9.4%. The N,P-GQDs showed a fast response to NO2

good linear relationship with concentration of NO2

Liu and co-workers prepared N,P-GQDs as fluorescence sensor for the detection of nitrite with high sensitivity and high selectivity [54]. The N,P-GQDs were synthesized by hydrothermal method using tetrakis(hydroxymethyl)phosphonium chloride and ethylenediamine endcapped polyethylenimine as phosphorus, carbon, and nitrogen source, respectively. The N,P-GQDs were prepared by using different temperatures (230° and 250°) and showed higher oxygen, nitrogen, and phosphorus levels at 230° compared to the those at 250°. The absolute quantum yield

sensitivity and high selectivity. The fluorescence quenching efficiency exhibited a

limit was 2.5 nM. The results of MTT assays displayed over 90% cell viability by incubating N,P-GQDs with T24 cells for 24 h, suggesting good biocompatibility and

Ananthanarayanan et al. used carbonization strategy for the preparation of N,P-GQDs from biomolecule adenosine triphosphate (ATP) as nitrogen and phosphorus source [55]. Firstly, adenosine triphosphate (ATP) was carbonized for 1 h at 90° and got carbonized ATP; then the carbonized ATP was exfoliated in HNO3 for 24 h and got final product N,P-GQDs. The results of Raman spectrum characterization of carbonized ATP exhibited prominent D and G bands, indicative of the presence

 carbon with graphitic nature. The N,P-GQDs have many advantages, such as excellent biocompatibility, good photostability, high fluorescence quantum yield (QY ∼ 27.5% by calculation and ∼53.0% after chemical reduction using NaBH4), and low molecular weight (∼1.4 kDa). The doping proportions of N and P are

<sup>−</sup> with high

<sup>−</sup> within 5–30 nM. The detection

The hydrogen peroxide (H2O2) holds an important role in the biological system and is closely related with many diseases such as cancer, Parkinson disease, and so on [51]. The Pd nanoparticles decorated with N-GQDs @N-carbon hollow nanospheres was designed and synthesized as a high electrochemical sensor for the hydrogen peroxide detection [52]. The proposed NGQD@NC@Pd HNSs sensor showed highly efficient electrocatalytic activity as non-enzymatic catalyst for the reduction of H2O2. The NGQD@NC@Pd/GCE exhibited excellent repeatability and reproducibility by detecting eight different NGQD@NC@Pd/GCE in fixed concentration H2O2 with relative standard deviation (RSD) 2.7 and 3.6%, respectively. The cytotoxicity of NGQD@NC@Pd/GCE was evaluated by using Cell Counting Kit-8 (CCK8) assay. The results of CCK-8 assay displayed over 95% viability incubating NGQD@NC@Pd/GCE using MDA-MB-231 and HBL-100 cells for 4 h, indicating

*DOI: http://dx.doi.org/10.5772/intechopen.85136*

tion limit was 0.03 mg/L.

biocompatibility.

*nanosensors*

imaging nitrite in live cell.

*Nanorods and Nanocomposites*

anti-interference capability.

chlorine (Cl).

was 1 .3 nmol/L.

electrode showed excellent electrical conductivity and displayed sensitive response to DA. The modified GCE showed a good linear relationship with the concentrations within 1–150 μM. The detection limit was 0.115 μM (S/N = 3). The obtained GQD-NHCH2CH2NH2 functionalized GCE displayed good stability and excellent

The chemical doping is a common strategy and used for tailoring the properties of GQDs. The heteroatom-doped GQDs showed exceptional properties such as tunable emission, changeable spin density, and charge distribution of carbon atoms [45]. Dopants include N, sulfur (S), phosphorus (P), boron (B), fluorine (F), and

The first successful synthesis of nitrogen-doped GQDs was reported by Li and co-workers in 2012 [46]. Liu et al. synthesized N-GQDs by hydrothermal method using citric acid as carbon sources and ammonia as nitrogen sources with N/C atomic ratio of ca. 4.3% emitting an obviously blue luminescence [47]. The fluorescence quantum yield of N-GQDs was 2.46% by calculation. The as-prepared N-GQDs can strongly adsorb 18 mer ssDNA (5′-ATACCAGCTTATTCAATT-3′) via π-π interaction force. The fluorescence of N-GQDs was quenched by photo-induced electron transfer mechanism between N-GQDs and ssDNA. The fluorescence of N-GQDs can be recovered upon addition of mixture of bleomycin and Fe(II) due to

Fan and co-worker constructed N-GQD-Hg(II) complex system as a highly sensitive fluorescence sensor for cysteine detection [48]. The N-GQDs was prepared by one-pot method using citric acid as carbon source and urea as nitrogen sources. The N-GQD-Hg(II) complex as fluorescence sensor showed weak fluorescence. The fluorescence was recovered upon addition of cysteine to the complex system of N-GQD-Hg(II) due to the coordinate interaction between cysteine and Hg(II). The fluorescence intensity showed good linear relationship with the concentration of cysteine within a range of 0.05–30 μmol/L. The detection limit

Zhao et al. prepared oxygen-rich nitrogen-doped GQDs by using one-pot synthesis strategy as pH-sensitive sensor for the detection of Hg(II) ion [49]. The oxygen-rich N-GQDs were synthesized by using citric acid (CA) and 3,4-dihydroxyl-phenylalanine (l-DOPA) as the carbon source and the N source, respectively. The N-GQDs showed excitation-wavelength-independent fluorescent behavior, and the quantum yield was 18%. The N-GQDs as an efficient fluorescent sensor displayed the highly sensitivity and highly selectivity for the detection of Hg(II) based on the mechanism of nonradiative electron transfer. The detection limit was 8.6 nM. The fluorescence quenching efficiency showed good linear relationship with the concentration of Hg(II) within concentration from 0.04 to 6 μM. The competitive experiments showed that the N-GQDs showed high selectivity and sensitivity for

the detection of Hg(II) even in the interference of other metal ions.

The strip-based fluorescence molecularly imprinted sensor was designed and constructed for monitoring thiacloprid [50]. The fluorescence molecularly imprinted sensor was synthesized based on polydopamine (PDA) polymer, thiacloprid, and N-GQDs. Firstly, the filter paper is dipped into N-GQD aqueous solution; secondly, the dopamine with thiacloprid self-polymerized on the surface of strip. The polydopamine molecularly imprinted polymer acted as an high efficient

**2.5 Nanocomposite-based doped graphene quantum dots as nanosensors**

*2.5.1 Nitrogen-doped graphene quantum dots (N-GQDs) as nanosensors*

the noncovalent binding between bleomycin and ssDNA.

**184**

sensor for the detection of thiacloprid. The as-prepared fluorescence molecularly imprinted sensor showed a linear relationship between 0.1 and 10 mg/L, and detection limit was 0.03 mg/L.

The hydrogen peroxide (H2O2) holds an important role in the biological system and is closely related with many diseases such as cancer, Parkinson disease, and so on [51]. The Pd nanoparticles decorated with N-GQDs @N-carbon hollow nanospheres was designed and synthesized as a high electrochemical sensor for the hydrogen peroxide detection [52]. The proposed NGQD@NC@Pd HNSs sensor showed highly efficient electrocatalytic activity as non-enzymatic catalyst for the reduction of H2O2. The NGQD@NC@Pd/GCE exhibited excellent repeatability and reproducibility by detecting eight different NGQD@NC@Pd/GCE in fixed concentration H2O2 with relative standard deviation (RSD) 2.7 and 3.6%, respectively. The cytotoxicity of NGQD@NC@Pd/GCE was evaluated by using Cell Counting Kit-8 (CCK8) assay. The results of CCK-8 assay displayed over 95% viability incubating NGQD@NC@Pd/GCE using MDA-MB-231 and HBL-100 cells for 4 h, indicating good biocompatibility of the NGQD@NC@Pd/GCE.

Peng and co-workers designed and reported a strategy method to detect Hg(II) ions by accelerating reaction rate between porphyrin and Mn(II) based on synergistic effect of N-GQDs and Hg(II) [53]. The reaction mechanism is based on larger Hg(II) of porphyrin-Hg(II) complex, which was replaced by smaller Mn(II) ions forming porphyrin-Mn(II) complex in a relatively faster speed. Such course was accompanied by the absorption red-shift and fluorescence quenching of porphyrins; meantime, the fluorescence intensity of N-GQDs enhanced. The CCK-8 assay showed over 90% viability by incubating 5.0 μM TMPyP, 40 μM Mn(II), or 20 μg/L N-GQDs for 24 h using A549 cells, indicative of good biocompatibility.
