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A xanthene-based colorimetric sensor, 2-((5-chloro-2-oxoindolin-3-ylidene)amino)-3',6'-bis(diethylamino)spiro[isoindoline-1,9'-xanthen]-3-one, was designed and its metal sensing properties was evaluated in aqueous solutions. The sensor showed colorimetric response toward Hg2+ from colorless solution to pink among various metal ions such as Na+, K+, Mg2+, Ca2+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Al3+, Cd2+ and Pb2+. The addition of Hg2+ exhibit an absorption enhancement of the sensor based on a spirocycle ring‐opening process and a subsequent hydrolysis reaction. The detection limit of the sensor for Hg2+ was found to be 7.88×10−8 M.

A novel, simple, sensitive and rapid spectrophotometric method for the determination of selenium(iv) in an acidic medium using rhodamine B hydrazide (RBH) has been developed. The method is based on the micellar mediated oxidation of RBH by Se(iv) in an acidic medium to produce a pinkish violet color of rhodamine B, which was monitored spectrophotometrically at λmax 585 nm. The sensitivity of the method was found to increase when the reaction was performed in a micellar medium. Beer's law was obeyed in the concentration range of 0.002-0.032 μg mL-1. The reaction variables such as time, temperature, reagent concentration, and acidity have been optimized for the reaction. Sandell's sensitivity and molar absorptivity for the reaction system were found to be 0.00004 μg cm-2 and 42.52 × 106 L mol-1 cm-1, respectively. The developed method was successfully applied for the determination of Se(iv) in several real samples with quantitative results.

Upconversion nanoparticles (UCNPs) are widely used as energy donors via a luminescence resonance energy transfer (LRET) technique in biosensing field. To enhance LRET efficiency and afford an improved sensitivity for Cu2+ detection, sandwich structured NaYF4@NaYF4:Er3+/Yb3+@NaYF4 UCNPs have been designed and prepared. For the core-internal shell-external shell structure, luminescence ions (Er3+) are located in the internal shell near the nanoparticle surface, which is close enough to energy acceptors outside. To study the sensing properties, the UCNPs are used as energy donors and rhodamine B hydrazide (RBH) as energy acceptors to construct the nanoprobes. It is found that the sandwich structured nanoparticles gains more intense luminescence than the naked NaYF4:Er3+/Yb3+ UCNPs, which is beneficial to the further improvement of signal sensitivity in biological detection. Moreover, the specificity for detecting Cu2+ is also checked and found that the designed probe is much more sensitive to Cu2+ than the other ions including K+, Na+, Li+, Ca2+, Ba2+, Mg2+, Zn2+ and Sr2+. The experiment on the stability of UCNPs-RBH probe and RBH shows the 980 nm laser produces less photodamage to RBH than that of visible light. Therefore, the UCNPs can be used to construct potential probes for realizing effective detection of Cu2+.

In present work, rhodamine B hydrazide and rhodamine 6G hydrazide were synthesized and the chemiluminescence performance has been investigated. Based on the chemiluminescence of rhodamine 6G hydrazide‑chromium(VI), a selective and sensitive method for the direct detection of chromium(VI) was developed. The chemiluminescence intensity was linearly related to the concentration of chromium(VI) in the range of 2.60 × 10− 8–8.00 × 10− 6 mol/L with a correlation coefficient of r = 0.998 and a detection limit of 1.4 × 10− 8 mol/L (S/N = 3). The results indicated rhodamine 6G hydrazide was an excellent chemiluminescent probe for chromium(VI) without reduction of chromium(VI) to chromium(III). A possible mechanism of CL emission was also suggested.

In this article, we successfully designed and synthesized a novel chemosensor PW bearing rhodamine B hydrazide and 8-formyl-7-hydroxyl-4-methylcoumarin, which displayed both colorimetric and “turn-on” fluorescence responses for CN− in DMSO/H2O (1 : 1, v/v, pH = 7.20) solution. The probe could distinguish CN− via a deprotonation process and the mechanism of intramolecular charge transfer (ICT), which was proved by 1H NMR titration, ESI-MS and DFT calculations. The detection limit of the sensor PW towards CN− was 1.592 × 10−7 M. Moreover, the sensor PW was successfully utilized to detect CN− in bitter almond. Test strips containing PW were also prepared, which could act as a colorimetric tool to detect CN− in practical application.

A tetraphenylethene functionalized rhodamine sensor RbTPE has been synthesized by the condensation of tetraphenylethene-aldehyde and rhodamine B hydrazide. RbTPE exhibited an aggregation-induced emission effect in the THF/water system, and displayed high selectivity and sensitivity for HCl vapor in the solid state. The solid-state emission of RbTPE can be reversibly switched between blue and red due to the stronger protonation of the rhodamine amino group by HCl. A satisfactory nonlinear relationship was observed upon exposure to HCl vapor in the range of 0–500 ppm, and the detection limit was as low as 20 ppm. After exposing the post-test product RbTPE–HCl to NH3 vapor for a few minutes, the spirolactam ring closed and the signals restored, which showed that RbTPE could be used as a “naked-eye” and reversible sensor for detecting HCl and NH3.

Rhodamine B hydrazide (RBH) was prepared as a new derivatizing reagent for residual formaldehyde in a drug such as N-butyl-2-cyanoacrylate (NBCA). RBH reacted with formaldehyde rapidly at room temperature, and the resulting derivative can be detected sensitively at 238 nm. After optimizing the derivatization and chromatographic separation conditions, RBH as a pre-column derivatizing reagent has been successfully used for the analysis of formaldehyde in real NBCA samples by high-performance liquid chromatography (HPLC). The detection limit of the method is 0.004 μg/mL for formaldehyde, and the recovery is 94.0%-96.6%. Most notably, compared with the previous HPLC methods, the developed method has the advantages of mild derivatization temperature, short derivatization time and high sensitivity.

N-acryloyl rhodamine B hydrazide, a non-responsive control of a colorimetric Cu(2+) sensor, was used as a turn-on fluorescent sensor for Hg(II) from HgCl2 in the presence of AgNO3. The detection was highly selective and sensitive, and a large number of environmentally and biologically relevant metal ions, such as Na(+), K(+), Ca(2+), Mg(2+), Fe(3+), Cu(2+), Zn(2+), Cr(3+), Pb(2+), Ni(2+), Fe(2+), Mn(2+), Co(2+), Cd(2+), including Hg(II) from the easy dissociated salts, did not show significant interference. The fluorescence of the sensor (10 μM) was enhanced 74 folds by 10 equiv. of Hg(II) from HgCl2 in THF/HAc-NaAc (1/1, v/v, pH = 6) aqueous buffer solution containing 20 equiv. of AgNO3. The maximal fluorescence intensity increased linearly with the concentration of Hg(II) in the range of 0-70 μM. The detection limit of Hg(II) was 0.59 μM. The sensing mechanism was explored by Job's plot experiment, reversible experiment, mass spectrum analysis, spectroscopic analysis, and thin-layer chromatography.

A bifunctional colorimetric and fluorescent chemosensor RbCS has been successfully designed and synthesized by simple condensation of 3,5-dichlorosalicylaldehyde and rhodamine B hydrazide. In aqueous media, chemosensor RbCS exhibits remarkably enhanced absorbance intensity and color change from colorless to pink for Cu 2+ , and shows significant “off–on” fluorescence accompanied with color changes from colorless to red upon binding to Hg 2+ . The absorbance and fluorescence signals of RbCS can be restored with addition of EDTA disodium into solutions of RbCS–Cu 2+ and RbCS–Hg 2+ , showing that the binding of chemosensor RbCS and Cu 2+ /Hg 2+ is chemically reversible.

A novel rhodamine-based chemosensor, 3′,6′-bis(diethylamino)-2-((2,7-dimethoxy-9H-fluoren-9-ylidene)amino)spiro[isoindoline-1,9′-xanthen]-3-one (1) for aluminum ion, was designed and synthesized. Compound 1 is an orange colored, weak fluorescent compound which was synthesized via one-step facile reaction of rhodamine B hydrazide with 2,7-dimethoxy-9H-fluoren-9-one in MeOH. When Al3+ salt was added in methanol:water (30:70, v/v) solution then spirolactum ring of 1 was opened. The absorbance and fluorescence of the mixed solution were increased dramatically because of the opening of the lactum ring. Thus, signal transduction occurred via reversible CHEF (chelation-enhanced fluorescence) mechanism in the range 580–584 nm.

A new rhodamine B derivative colorimetric and fluorescent sensor (1) was synthesized by condensation reaction of rhodamine B hydrazide and 2,4-dihydroxybenzaldehyde, which showed reversible and highly selective and sensitive recognition toward Cu2+ over other examined metal ions. Upon addition of Cu2+, sensor 1 exhibits remarkably enhanced absorbance intensity and color change from colorless to pink in DMSO and MeCN aqueous buffer solution or pure MeCN, and shows significant off–on fluorescence accompanied by color changes from colorless to orange in MeCN. The sensor 1 was also successfully applied to the determination of Cu2+ in water samples.

A novel spectrofluorimetric method for the determination of sulfite using rhodamine B hydrazide as fluorogenic reagent in the presence of polyoxyethylene sorbitan monooleate (Tween 80) surfactant micelles is described. The method is based on the mixture of sulfite with rhodamine B hydrazide, a colorless, non-fluorescent substance in Tween 80 surfactant micelles which gives rhodamine B-like fluorescence emission. The fluorescence intensity increase is linearly related to the concentration of sulfite in the range 5–800 ng ml −1 with a detection limit of 1.4 ng ml −1 (3 σ ). The optimal conditions for the detection of sulfite are evaluated and the possible detection mechanism is also discussed. The method has been successfully applied to the determination of total sulfite in wines and compares well with the standard iodimetric method.

A new chemosensor has been used to monitor atmospheric nitrogen oxides [NO + NO2] at parts per billion (ppb) level. It is based on the catalytic reaction of nitrogen oxides with rhodamine B hydrazide (RBH) to produce a colored compound through the hydrolysis of the amide bond of the molecule. A simple colorimeter has been used to monitor atmospheric nitrogen dioxide at ppb level. The air samples were purged through a sampling cuvette containing RBH solution using peristaltic pump. The proposed method has been successfully applied to monitor the ambient nitrogen dioxide levels at traffic junction points within the city limits and the results obtained are compared with the standard Griess-Ilosvay method.

A novel method for the determination of peroxynitrite using rhodamine B hydrazide as a fluorogenic probe is described. The method is based on the oxidation of rhodamine B hydrazide, a colorless, non-fluorescent substance, by peroxynitrite to give rhodamine B-like fluorescence emission. The fluorescence increase is linearly related to the concentration of peroxynitrite in the range of 7.5×10 −8 –3.0×10 −6 mol l −1 with a detection limit of 2.4×10 −8 mol l −1 (3 σ ). The optimal conditions for the detection of peroxynitrite were evaluated and the possible detection mechanism was also discussed in this paper.

A novel rhodamine B derivative L was synthesized facilely by the condensation reaction of rhodamine B hydrazide and 2-hydroxyquinoline-3-carbaldehyde, and its structure was characterized by H-1 NMR, MS techniques and elemental analysis. The recognition properties of the target compound L with metal ions had been investigated in CH3CN-H2O (V : V=1 : 1) solution by the ultraviolet spectrophotometry and the fluorescence spectrophotometry. The results showed that the probe L exhibited the selective colorimetric recognition of Cu2+ and the fluorogenic recognition of Hg2+. Upon the addition of Cu2+ or Hg2+, the spirolactam ring of probe L was opened and 1 : 1 ligand-copper complex or 1 : 2 ligand-mercury complex was formed, and both of the Cu2+ and Hg2+ recognition processes were proven to be a revisible process.

A simple, sensitive, and rapid spectrophotometric method is proposed for the determination of the trace amount of thallium. The method is based on the oxidation of rhodamine B hydrazide by thallium(I) in acidic medium to give a pinkish violet radical cation, having a maximum absorption at 565 nm. The sensitivity of the method was found to increase when performed in micellar medium. Beer's law is valid over the concentration range of 0.005 μg mL−1 to 6.4 μg mL−1 with molar absorptivity and Sandell's sensitivity of 6.305 × 104 L mol−1 cm−1 and 0.008 mg cm−1 respectively. The tolerance limits for interfering ions are discussed. All variables were studied in order to optimize the reaction conditions. The validity of the method was checked by its simultaneous determination in synthetic samples.