Journal of Pharmaceutical and Biomedical Analysis 48 (2008) 1064–1069 Contents lists available at ScienceDirect Journal of Pharmaceutical and Biomedical Analysis journal homepage: www.elsevier.com/locate/jpba Cyclodextrin-based potentiometric sensors for midazolam and diazepam C.G. Amorim a , A.N. Araújo a , M.C.B.S.M. Montenegro a,∗ , V.L. Silva b a REQUIMTE, Servic ¸o de Química-Física, Fac. Farmácia (U.P.), Rua Aníbal Cunha 164, 4099-030 Porto, Portugal b Dep. Engenharia Química (DEQ), Universidade Federal de Pernambuco (UFPE), Av. Prof. Artur de Sá, Cidade Universitária, s/n 50740-521, Recife-PE, Brazil article info Article history: Received 10 April 2008 Received in revised form 7 August 2008 Accepted 11 August 2008 Available online 22 August 2008 Keywords: Cyclodextrins Ion-selective electrodes Benzodiazepines Lab-on-valve abstract In this work the implementation of benzodiazepine ion-selective electrodes for pharmaceutical formu- lations control is described. The solid-contact electrodes for midazolam and diazepam are based on polymeric membranes incorporating respectively -cyclodextrin and (2-hydroxiproyl)--cyclodextrin as ionophores, 2-fluorophenyl 2-nitrophenyl ether as plasticizer and potassium tetrakis (p-chlorophenyl) borate as ionic additive. For conventionally shaped midazolam electrode a slope of 61.9 ± 1.3 mV dec -1 ,a LLLR of 5.7 ± 2.7 × 10 -4 gL -1 and pH range of 2.6–5.4 was obtained, while the corresponding values for diazepam electrodes were of 67.6 ± 3.0 mV dec -1 , 4.9 ± 1.5 × 10 -2 gL -1 and 1.9–2.7 pH units, respectively. Membrane optimization was based on the molar ratio between the ionophore and additive for midazo- lam and on inclusion cavity of cyclodextrin for diazepam. The miniaturization of the above-described electrodes gave rise to potentiometric detectors for sequential-injection lab-on-valve system with similar characteristics albeit the useful lifetime shortened from 1 year to approximately 15 days under continuous operation. The optimized flow conditions were achieved for sample injection volumes of 20 L propelled towards the detection cell at the flow rate of 16 Ls -1 during 80 s. Real sample analysis revealed statistical accuracy and between-days precision comparable to the general used chromatographic-based procedure. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Benzodiazepines are psychoactive therapeutic drugs with varying hypnotic, sedative, anxiolytic, anticonvulsant, mus- cle relaxant and amnesic properties, which are brought on by slowing down the activity of the central nervous sys- tem. Two representative benzodiazepines are midazolam and diazepam. Midazolam (8-chloro-6-(2-fluorophenyl)-1-methyl-4H- imidazo[1,5a][1,4]benzodiazepine) [1] is indicated for the acute management of aggressive or delirious patients and less for the acute management of seizures such as status epilepticus. Occasion- ally is used as a hypnotic, especially in hospitals. Diazepam (7-chloro-1-methyl-5-phenyl-3H-1,4-benzodia- zepin-2(1H)-one) [1] is mainly used to treat anxiety, insomnia, and symptoms associated with the acute alcohol or opiate with- drawal. It also induces sedation, anxiolysis or amnesia prior to certain medical procedures. These applications gained popularity among medical professionals, increasing the number of pharma- ceutical preparations in the market and consequently a need of development of different technics for quality control. ∗ Corresponding author. E-mail address: mcbranco@ff.up.pt (M.C.B.S.M. Montenegro). Spectrophotometric [2,3] and adsorptive stripping voltammet- ric [4] procedures are indicated in the literature for midazolam determinations in pharmaceuticals. Other techniques has been used for the diazepam determinations such as the direct square-wave (SWV) and square-wave cathodic stripping (SWCSV) voltammetries using the hanging mercury drop electrode [5], spectrophotometry [6,7], capillary gas chromatography with a nitrogen–phosphorus detector [8], reversed-phase liquid chro- matography and capillary electrophoresis [9]. The documented potentiometric procedures for diazepam are based on the use of solid-contact-ion-selective electrodes (ISE) with polyvinylchlo- ride membrane supporting an electroactive ion-pair complex, such as tetraphenylborate–diazepam or phosphotungstate–diazepam [10]. The best working characteristics are obtained after pre- vious conditioning for 24 h with an appropriate solution of the analyte. A similar approach was previously described by Nie et al. [11] by proposing ion-pair complexes such as tetraphenylborate–diazepam, dipicrylaminate–diazepam, tetra- iodobismuthate–diazapam or tetra-iodomercurate–diazepam for the development of selective electrodes with internal reference solution. From these works however several drawbacks could be pointed out to the use of ion-pair as electroactive material. The elec- trodes presented short lifetimes, they need to be implemented in a configuration that includes the use of an internal reference solution and require a long conditioning time (24 h) in the analyte concen- 0731-7085/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jpba.2008.08.012