*2.3.3 Detection of inorganic ions*

The sulfanilic acid and glutathione-functionalized GQDs was constructed and synthesized as fluorescent sensor for the detection of sulfide anions and ascorbic acid [26]. The sulfanilic acid and glutathione-functionalized GQDs were prepared through amide linkage using 1-(3-dimethylaminopropyl)-3-ethyl-carbodiimide hydrochloride (EDC) as catalyst, namely, SSGQDs. The SSGQDs showed strong fluorescence property. The fluorescence of SSGQDs was quenched upon addition of Cu2+ ions, forming SSGQD-Cu(II) complex. The S2<sup>−</sup> ions showed high coordination interaction with Cu2+ ions from SSGQD-Cu(II) complex and induced the fluorescence recovery of SSGQDs. The ascorbic acid (AA) as a reduction can reduce Cu2+ into Cu+ and induced the disaggregation of the SSGQDs, and fluorescence of

**181**

*Nanocomposite-Based Graphene for Nanosensor Applications*

been applied to detect chlorine in drinking water.

detect Cu2+ and l-cysteine in serum samples.

environmental monitoring and biomedical applications.

concentrations of Hg2+ ions within 2.0 × 10−<sup>6</sup>

*2.3.4 Detection of proteins*

SSGQDs was recovered again. The GQDs as a green sensor were synthesized for the detection of free chlorine with high selectivity and high sensitivity [27]. The GQDs showed strong fluorescence property, and fluorescence of GQDs was quenched upon addition of chlorine-based fluorescence resonance energy transfer. The fluorescence quenching efficiency exhibited a good linear relationship with concentrations of chlorine with a wide range from 0.05 to 10 μM. The sensing system has

The europium-functionalized GQDs (Eu-GQDs) were synthesized by treatment of Eu-decorated graphene (3D Eu-graphene) through a strong acid oxidation [28]. The amount of Eu was 2.54%. The Eu-GQDs complex showed higher electron density and surface chemical activities compared to that of GQDs. The as-synthesized Eu-GQDs exhibited a sensitive response for the detection of Cu2+ and l-cysteine with high selectivity and high sensitivity. The fluorescence of Eu-GQDs was quenched upon addition of Cu2+ due to the coordination interaction between Cu2+ and carboxyl groups of Eu-GQDs. The fluorescence of Eu-GQDs was recovered in the presence of l-cysteine due to strong affinity of Cu2+ and S of L-cysteine. The good linear relationship was shown within the range of 0.1–10 μM for Cu2+ and 0.5–50 μM for l-cysteine, respectively. The detection limit was 0.056 μM for Cu2+ and 0.31 μM for l-cysteine, respectively. The proposed nanosensor can be used to

A fluorescence sensor based on gold nanoparticles-functionalized GQDs has been designed and synthesized for the detection of Pb2+ with high sensitivity and high selectivity [29]. The GQDs showed strong fluorescence property. The fluorescence of GQDs was quenched in the presence of Au nanoparticles due to the aggregation of GQDs. The fluorescence of GQDs was recovered upon addition of Pb2+ ions inducing de-aggregation of gold nanoparticles-GQD complex. The fluorescence intensity exhibited a good linear relationship with the concentrations of Pb2+ ions within 50 nM–4 μM. The detection limit was 16.7 nM. The dopaminefunctionalized GQDs (DA-GQDs) was constructed and prepared for the detection of Fe3+ ions with high sensitivity and high selectivity [30]. The DA-GQDs showed bright blue fluorescence, and the fluorescence of DA-GQDs was quenched in the presence of Fe3+ ions. The fluorescence quenching efficiency exhibited a good linear relationship with the concentrations of Fe3+ ions between 20 nM and 2 μM. The detection limit was 7.6 nM. The DA-GQD sensing probe displayed excellent selectivity for the detection of Fe3+ ions in the presence of other biomolecules. The reaction mechanism of Fe3+ was based on coordination interaction and oxidation of dopamine. The as-synthesized nanosensor as sensing platform can be widely used for

The folic acid-functionalized GQDs (FA-GQDs) were designed and synthesized by thermal pyrolysis of maleic acid (MA) and folic acid (FA) [31]. The FA-GQDs showed obvious fluorescence behavior, and fluorescence property depends on the different ratio of FA/MA used in thermal pyrolysis. The FA-GQDs as a turn-on fluorescent sensor showed a high sensitivity for the detection of folate receptor-positive cancer cells. The resulting FA-GQDs also exhibited a fluorescence response to Hg2+ ions. The fluorescence quenching efficiency showed a good linear relationship to the

was 1.7 × 10<sup>−</sup>12 M (S/N = 3). The FA-GQD nanosensor displayed excellent selectivity for the detection of Hg2+ ions in the presence of other metals and biomolecules.

The fluorescence sensor lecithin/β-CD@NR@ GQD complex was constructed and synthesized by covalence Nile red (NR) onto GQDs using

to 5.0 × 10<sup>−</sup>12 M. The detection limit

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

#### *Nanocomposite-Based Graphene for Nanosensor Applications DOI: http://dx.doi.org/10.5772/intechopen.85136*

*Nanorods and Nanocomposites*

*2.3.1 Detection of amino acids*

successfully applied to detect GSH in living cells.

*2.3.2 Detection of drug molecules*

104 μg/L. The detection limit was 1 μg/L.

*2.3.3 Detection of inorganic ions*

**sensors**

**2.3 Nanocomposite-based graphene quantum dots (GQDs) as fluorescence** 

The fluorescent graphene quantum dots-gold nanoparticles as nanosensor showed a high selectivity and high sensitivity for the detection of cysteine [23]. The AuNPs@r-GQDs nanocomposite was prepared by the following processes. First, nitrogen-doped graphene quantum dots (N-GQDs) were reduced to r-GQDs by NaBH4 as reductant and subsequently the r-GQDs converted HAuCl4 to Au nanoparticles (AuNPs) by reduction reaction and coated onto AuNPs forming core-shell-structured AuNPs@r-GQDs. The AuNPs@r-GQDs showed good dispersion behavior with an intensive surface plasma band at 525 nm. The AuNPs@r-GQDs exhibited aggregation behavior and led to their color change by using cysteine as cross-linking agent through adsorption of Ag ions onto their surface. The detection limit was 5.6 nM. Furthermore, the AuNPs@r-GQDs showed higher selectivity for cysteine than that of glutathione (GSH) even at the interfere condition of 1000-fold concentrations of GSH.

The GQD-MnO2 complex as a convenient fluorescence nanosensor has been constructed and prepared for the detection of glutathione (GSH) with high selectivity and high sensitivity [24]. The fluorescence intensity of GQDs was quenched upon addition of MnO2 nanosheet based on the mechanism of fluorescence resonance energy transfer (FRET). The fluorescent signal recovered upon GSH reducing MnO2 nanosheets into Mn2+ ions and releasing GQDs. The GQD-MnO2 complex as nanoprobe showed a sensitive response to GSH between 0.5 and 10 μmol/L. The fluorescence intensity showed a good linear relationship with the concentrations of GSH. The detection limit was 150 nmol/L. The GQD-MnO2 complex exhibited higher selectivity for the GSH than that of other metal ions and biomolecules and

Zhou et al. designed and developed a convenient fluorescent sensor based on the molecularly imprinted polymers (MIPs)-functionalized GQDs for the detection of tetracycline (TC) with high sensitivity and high selectivity [25]. The GQDs were prepared by one-pot method, and the amino-functionalized GQDs and carboxylfunctionalized GQDs were fabricated, respectively. The GQD-MIPs were synthesized by sol-gel method. The GQD-MIPs exhibited strong fluorescence property, and the fluorescence was quenched upon addition of TC. The fluorescence quench efficiency showed a linear relationship with concentrations of TC between 1.0 and

The sulfanilic acid and glutathione-functionalized GQDs was constructed and synthesized as fluorescent sensor for the detection of sulfide anions and ascorbic acid [26]. The sulfanilic acid and glutathione-functionalized GQDs were prepared through amide linkage using 1-(3-dimethylaminopropyl)-3-ethyl-carbodiimide hydrochloride (EDC) as catalyst, namely, SSGQDs. The SSGQDs showed strong fluorescence property. The fluorescence of SSGQDs was quenched upon addition of Cu2+ ions, forming SSGQD-Cu(II) complex. The S2<sup>−</sup> ions showed high coordination interaction with Cu2+ ions from SSGQD-Cu(II) complex and induced the fluorescence recovery of SSGQDs. The ascorbic acid (AA) as a reduction can reduce

and induced the disaggregation of the SSGQDs, and fluorescence of

**180**

Cu2+ into Cu+

SSGQDs was recovered again. The GQDs as a green sensor were synthesized for the detection of free chlorine with high selectivity and high sensitivity [27]. The GQDs showed strong fluorescence property, and fluorescence of GQDs was quenched upon addition of chlorine-based fluorescence resonance energy transfer. The fluorescence quenching efficiency exhibited a good linear relationship with concentrations of chlorine with a wide range from 0.05 to 10 μM. The sensing system has been applied to detect chlorine in drinking water.

The europium-functionalized GQDs (Eu-GQDs) were synthesized by treatment of Eu-decorated graphene (3D Eu-graphene) through a strong acid oxidation [28]. The amount of Eu was 2.54%. The Eu-GQDs complex showed higher electron density and surface chemical activities compared to that of GQDs. The as-synthesized Eu-GQDs exhibited a sensitive response for the detection of Cu2+ and l-cysteine with high selectivity and high sensitivity. The fluorescence of Eu-GQDs was quenched upon addition of Cu2+ due to the coordination interaction between Cu2+ and carboxyl groups of Eu-GQDs. The fluorescence of Eu-GQDs was recovered in the presence of l-cysteine due to strong affinity of Cu2+ and S of L-cysteine. The good linear relationship was shown within the range of 0.1–10 μM for Cu2+ and 0.5–50 μM for l-cysteine, respectively. The detection limit was 0.056 μM for Cu2+ and 0.31 μM for l-cysteine, respectively. The proposed nanosensor can be used to detect Cu2+ and l-cysteine in serum samples.

A fluorescence sensor based on gold nanoparticles-functionalized GQDs has been designed and synthesized for the detection of Pb2+ with high sensitivity and high selectivity [29]. The GQDs showed strong fluorescence property. The fluorescence of GQDs was quenched in the presence of Au nanoparticles due to the aggregation of GQDs. The fluorescence of GQDs was recovered upon addition of Pb2+ ions inducing de-aggregation of gold nanoparticles-GQD complex. The fluorescence intensity exhibited a good linear relationship with the concentrations of Pb2+ ions within 50 nM–4 μM. The detection limit was 16.7 nM. The dopaminefunctionalized GQDs (DA-GQDs) was constructed and prepared for the detection of Fe3+ ions with high sensitivity and high selectivity [30]. The DA-GQDs showed bright blue fluorescence, and the fluorescence of DA-GQDs was quenched in the presence of Fe3+ ions. The fluorescence quenching efficiency exhibited a good linear relationship with the concentrations of Fe3+ ions between 20 nM and 2 μM. The detection limit was 7.6 nM. The DA-GQD sensing probe displayed excellent selectivity for the detection of Fe3+ ions in the presence of other biomolecules. The reaction mechanism of Fe3+ was based on coordination interaction and oxidation of dopamine. The as-synthesized nanosensor as sensing platform can be widely used for environmental monitoring and biomedical applications.

The folic acid-functionalized GQDs (FA-GQDs) were designed and synthesized by thermal pyrolysis of maleic acid (MA) and folic acid (FA) [31]. The FA-GQDs showed obvious fluorescence behavior, and fluorescence property depends on the different ratio of FA/MA used in thermal pyrolysis. The FA-GQDs as a turn-on fluorescent sensor showed a high sensitivity for the detection of folate receptor-positive cancer cells. The resulting FA-GQDs also exhibited a fluorescence response to Hg2+ ions. The fluorescence quenching efficiency showed a good linear relationship to the concentrations of Hg2+ ions within 2.0 × 10−<sup>6</sup> to 5.0 × 10<sup>−</sup>12 M. The detection limit was 1.7 × 10<sup>−</sup>12 M (S/N = 3). The FA-GQD nanosensor displayed excellent selectivity for the detection of Hg2+ ions in the presence of other metals and biomolecules.

#### *2.3.4 Detection of proteins*

The fluorescence sensor lecithin/β-CD@NR@ GQD complex was constructed and synthesized by covalence Nile red (NR) onto GQDs using

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

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 detection limit was 0.192 pg/mL.
