*2.5.3 Nitrogen and sulfur co-doped graphene quantum dots (N,S-GQDs) as nanosensors*

Mondal et al. designed and synthesized N,S-GQDs from the mixture of graphene oxide solution and thiourea by hydrothermal method [57]. The N,S-GQDs exhibited strong emission peak at 405 nm upon excitation at 320 nm. The N,S-GQDs showed a sensitive response to 2,4,6-trinitrophenol with highly selectivity, and the detection limit was 19.05 ppb. The fluorescence of N,S-GQDs significantly decreased upon addition of 2,4,6-trinitrophenol based on photo-induced electron transfer (PET) mechanism. The fluorescence quenching efficiency showed good linear relationship with concentration of 2,4,6-trinitrophenol. The N,S-GQDs showed higher quenching efficiency compared to these of N-GQDs, S-GQDs, and GQDs. Gavgani and co-workers constructed an ammonia (NH3) sensor based on N,S-GQDs/polyaniline (PANI) hybrid with high sensitivity and high selectivity [58]. The N,S-GQDs were synthesized by hydrothermal process using citric acid as carbon source and using thiourea as sulfur source, respectively. The N,S-GQDs/ PANI hybrid was prepared by using in situ chemical oxidative polymerization. The increased proportion of N,S-GQDs in N,S-GQDs/PANI hybrid showed considerable improvement of NH3 response, such as around 42% at 100 ppm and 385% at 1000 ppm, respectively. The N,S-GQDs/PANI hybrid showed fivefold higher response compared to that of free PANI. The enhancement of sensing properties for the N,S-GQDs/PANI hybrid attributed to the synergistic effect between the N,S-GQDs and PANI.

Chen et al. synthesized N,S-GQDs by one-pot pyrolysis method with quantum yield of 67% using citric acid and cysteine as carbon source and nitrogen and sulfur source, respectively [59]. The N,S-GQDs showed an excitation-independent emission property. The fluorescence of N,S-GQDs was quenched upon addition of AgNPs, and the fluorescence of N,S-GQD-AgNPs was recovered in the presence of CN<sup>−</sup>. The N,S-GQDs have no effect on the adsorption spectrum of AgNPs; however, addition of CN<sup>−</sup> obviously decreased the absorbance of AgNPs. The detection limit was 0.52 μM for fluorescent sensors and 0.78 μM for colorimetric sensors. The assynthesized N,S-GQD-AgNPs as nanosensor has been successfully applied to detect CN<sup>−</sup> in realized water samples.

**187**

*Nanocomposite-Based Graphene for Nanosensor Applications*

*2.5.4 Sulfur-doped graphene quantum dots (S-GQDs) as nanosensors*

The S-GQDs were prepared and reported by Bian and co-workers through one-pot hydrothermal method using compound 1,3,6-trinitropyrene as carbon source and 3-mercaptopropionic acid (MPA) as sulfur source [60]. The S-GQDs as fluorescent sensing probes showed highly sensitive response to the Ag+

with high selectivity within a wide linear range of 0.1–130.0 μM. The detection limit was 30 nM. The fluorescence intensity of S-GQDs was obviously decreased

transfer (PET) mechanism. The feasibility of as-synthesized S-GQDs as fluorescent

Li et al. synthesized the S-GQDs by electrochemical approach using graphite

In this chapter, we concluded recent development of modified graphene-based nanocomposites (including of GO, GQDs, doped GQDs) as novel and convenient fluorescence nanosensors and electrochemical sensors for the detection of amino acids, proteins, metal ions, inorganic anions, drug molecules, and small molecules, pH, respectively. The obtained functionalized fluorescence sensors and electrochemical sensors as sensing platforms displayed high sensitivity and high selectivity

The graphene-based composites attracted the interest of scientific workers due to its nanosize, quantum confinement, edge effects, low toxicity, and good conductivity. We comprehensively summarized the preparation, dopant element, interaction mechanism, and practical applications. The relationship between response signals and analyte concentrations was discussed in detail. The functionalized graphene-based nanocomposites showed good biocompatibility and low toxicity.

electrode in sodium p-toluenesulfonate aqueous solution [61]. The S-GQDs obviously improved surface chemistry and electronic properties. The S-GQDs as fluorescent sensor showed a sensitive response to the Fe3+ ions with high selectivity and high sensitivity. The fluorescence intensity of S-GQDs obviously decreased

upon addition of Fe3+ ions concentration between 0.01 and 0.70 μM. The fluorescence quenching efficiency showed good linear relationship with the concentration of Fe3+ ions. The detection limit was 4.2 nM. The S-GQDs as a fluorescent sensor can be reused over five times without signal lost. This fluorescence sensing probe can be successfully applied to detect Fe3+ ions in human serum. Dong et al. prepared S-GQDs by hydrothermal process using the mixture of 1,3,6-trinitropyrene, Na2S, and NaOH in aqueous solution [62]. The reported S-GQDs exhibited a stable yellow-green emission. It was found that fluorescence quenching was pH-dependent and showed best quenching efficiency at pH 7.0. The as-synthesized S-GQDs showed excitation-independent photoluminescence property. The S-GQDs as a newly fluorescent probe showed high selectivity and high sensitivity for the detection of Pb(II) ions. Compared to Pb(II) ions, ions such as Na(I), K(I), Cu(II), Ca(II), Mg(II), Fe(III), Ni(II), Co(II), and Cd(II) have no obvious effect on the fluorescence intensity of S-GQDs. The fluorescence intensity of S-GQDs significantly decreased upon addition of Pb(II) ions from 0.1 to 220.0 mM in aqueous solution. The fluorescence quenching efficiency showed good linear relationship with concentrations of Pb(II) from 0.1 to 140.0 μM. The

ions. The fluorescence quenching efficiency showed a good

ions based on photo-induced electron

detection in local lake.

ions

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

linear relationship with concentration of Ag+

sensing probe in practical application was assessed by Ag+

The detection results obtained from S-GQDs and ICP-MS were close.

upon addition of Ag<sup>+</sup>

detection limit was 0.03 μM.

for the detection of biomolecules, respectively.

**3. Conclusion**

*Nanorods and Nanocomposites*

ing N,P-GQDs with a transferrin.

ous alcohols by mild and green ways.

*nanosensors*

6.2 (C/N = 7.0) and 6.9 (C/P = 6.3), respectively. The N,P-GQDs exhibited good two-photon upconversion properties. The strong upconverted photoluminescence phenomenon was showed with maximum emission at ∼560 nm upon excitation at 800 nm. The lifetime measurements of N,P-GQDs exhibited τ1 (*A*1) and τ2 (*A*2) to be 320 ps (0.44) and 1.62 ns (0.56), respectively, where τ1 and τ2 are time constants and *A*1 and *A*2 are the corresponding amplitudes. The imaging and real-time tracking of transferrin receptors in human cervical cancer cells came true upon conjugat-

Mahyari and Gavgani designed and constructed cobalt porphyrin-supported N,P-GQDs/graphene (CoPP@N,P-GQDs/G) complex as noble metal-free photocatalysts [56]. Firstly, the N,P-GQDs were synthesized by carbonization of adenosine triphosphate as nitrogen source and phosphorous source; secondly, the N,P-GQDs were embedded on graphene oxide; and thirdly, the cobalt porphyrins with photoactive property were loaded through ionic interaction. The resultant product CoPP@N,P-GQDs/G showed good dispersion in the reaction medium (water). The CoPP@N,P-GQDs/G complex as recyclable photocatalysts showed high efficiency with the aerobic oxidation reaction of alcohols by using visible-light irradiation. Furthermore, CoPP@N,P-GQDs/G complex displayed the good selectivity for vari-

*2.5.3 Nitrogen and sulfur co-doped graphene quantum dots (N,S-GQDs) as* 

Mondal et al. designed and synthesized N,S-GQDs from the mixture of graphene oxide solution and thiourea by hydrothermal method [57]. The N,S-GQDs exhibited strong emission peak at 405 nm upon excitation at 320 nm. The N,S-GQDs showed a sensitive response to 2,4,6-trinitrophenol with highly selectivity, and the detection limit was 19.05 ppb. The fluorescence of N,S-GQDs significantly decreased upon addition of 2,4,6-trinitrophenol based on photo-induced electron transfer (PET) mechanism. The fluorescence quenching efficiency showed good linear relationship with concentration of 2,4,6-trinitrophenol. The N,S-GQDs showed higher quenching efficiency compared to these of N-GQDs, S-GQDs, and GQDs. Gavgani and co-workers constructed an ammonia (NH3) sensor based on N,S-GQDs/polyaniline (PANI) hybrid with high sensitivity and high selectivity [58]. The N,S-GQDs were synthesized by hydrothermal process using citric acid as carbon source and using thiourea as sulfur source, respectively. The N,S-GQDs/ PANI hybrid was prepared by using in situ chemical oxidative polymerization. The increased proportion of N,S-GQDs in N,S-GQDs/PANI hybrid showed considerable improvement of NH3 response, such as around 42% at 100 ppm and 385% at 1000 ppm, respectively. The N,S-GQDs/PANI hybrid showed fivefold higher response compared to that of free PANI. The enhancement of sensing properties for the N,S-GQDs/PANI hybrid attributed to the synergistic effect between the N,S-

Chen et al. synthesized N,S-GQDs by one-pot pyrolysis method with quantum

yield of 67% using citric acid and cysteine as carbon source and nitrogen and sulfur source, respectively [59]. The N,S-GQDs showed an excitation-independent emission property. The fluorescence of N,S-GQDs was quenched upon addition of AgNPs, and the fluorescence of N,S-GQD-AgNPs was recovered in the presence of CN<sup>−</sup>. The N,S-GQDs have no effect on the adsorption spectrum of AgNPs; however, addition of CN<sup>−</sup> obviously decreased the absorbance of AgNPs. The detection limit was 0.52 μM for fluorescent sensors and 0.78 μM for colorimetric sensors. The assynthesized N,S-GQD-AgNPs as nanosensor has been successfully applied to detect

**186**

GQDs and PANI.

CN<sup>−</sup> in realized water samples.

#### *2.5.4 Sulfur-doped graphene quantum dots (S-GQDs) as nanosensors*

The S-GQDs were prepared and reported by Bian and co-workers through one-pot hydrothermal method using compound 1,3,6-trinitropyrene as carbon source and 3-mercaptopropionic acid (MPA) as sulfur source [60]. The S-GQDs as fluorescent sensing probes showed highly sensitive response to the Ag+ ions with high selectivity within a wide linear range of 0.1–130.0 μM. The detection limit was 30 nM. The fluorescence intensity of S-GQDs was obviously decreased upon addition of Ag<sup>+</sup> ions. The fluorescence quenching efficiency showed a good linear relationship with concentration of Ag+ ions based on photo-induced electron transfer (PET) mechanism. The feasibility of as-synthesized S-GQDs as fluorescent sensing probe in practical application was assessed by Ag+ detection in local lake. The detection results obtained from S-GQDs and ICP-MS were close.

Li et al. synthesized the S-GQDs by electrochemical approach using graphite electrode in sodium p-toluenesulfonate aqueous solution [61]. The S-GQDs obviously improved surface chemistry and electronic properties. The S-GQDs as fluorescent sensor showed a sensitive response to the Fe3+ ions with high selectivity and high sensitivity. The fluorescence intensity of S-GQDs obviously decreased upon addition of Fe3+ ions concentration between 0.01 and 0.70 μM. The fluorescence quenching efficiency showed good linear relationship with the concentration of Fe3+ ions. The detection limit was 4.2 nM. The S-GQDs as a fluorescent sensor can be reused over five times without signal lost. This fluorescence sensing probe can be successfully applied to detect Fe3+ ions in human serum. Dong et al. prepared S-GQDs by hydrothermal process using the mixture of 1,3,6-trinitropyrene, Na2S, and NaOH in aqueous solution [62]. The reported S-GQDs exhibited a stable yellow-green emission. It was found that fluorescence quenching was pH-dependent and showed best quenching efficiency at pH 7.0. The as-synthesized S-GQDs showed excitation-independent photoluminescence property. The S-GQDs as a newly fluorescent probe showed high selectivity and high sensitivity for the detection of Pb(II) ions. Compared to Pb(II) ions, ions such as Na(I), K(I), Cu(II), Ca(II), Mg(II), Fe(III), Ni(II), Co(II), and Cd(II) have no obvious effect on the fluorescence intensity of S-GQDs. The fluorescence intensity of S-GQDs significantly decreased upon addition of Pb(II) ions from 0.1 to 220.0 mM in aqueous solution. The fluorescence quenching efficiency showed good linear relationship with concentrations of Pb(II) from 0.1 to 140.0 μM. The detection limit was 0.03 μM.

### **3. Conclusion**

In this chapter, we concluded recent development of modified graphene-based nanocomposites (including of GO, GQDs, doped GQDs) as novel and convenient fluorescence nanosensors and electrochemical sensors for the detection of amino acids, proteins, metal ions, inorganic anions, drug molecules, and small molecules, pH, respectively. The obtained functionalized fluorescence sensors and electrochemical sensors as sensing platforms displayed high sensitivity and high selectivity for the detection of biomolecules, respectively.

The graphene-based composites attracted the interest of scientific workers due to its nanosize, quantum confinement, edge effects, low toxicity, and good conductivity. We comprehensively summarized the preparation, dopant element, interaction mechanism, and practical applications. The relationship between response signals and analyte concentrations was discussed in detail. The functionalized graphene-based nanocomposites showed good biocompatibility and low toxicity.

The as-synthesized fluorescence sensors and electrochemical sensors have been successfully applied for the monitoring of biomolecules in the human serum samples. The graphene-based nanocomposites showed great potential as sensing platforms for the biomedical applications and clinic research.
